i. theor. BioE. (I977) 68, 95-100
Can a Reverse Transcriptuse be Involved in Centriole Duplication? HANS A. WENT Department of Zoology, Washington State University, Pullman, Washington 99164, U.S.A. (Received 2 November 1976, and in revisedform
18 February
1977)
Accepting tentatively that the nucleic acid thought to be present in centrioles and basal bodies is RNA rather than DNA, which had earlier been thought to be the case, it is suggested that this RNA represents the primary genome of the organelle which is replicated via a DNA intermediate with the aid of an RNA-instructed DNA polymerase (reverse transcriptase) into more RNA during the course of the duplication cycle. This suggestion is presented as an alternate, and not as a replacement, to one made earlier which states that the organellar RNA is a transcript of the primary DNA genome located in the nucleus. In support of this hypothesis are data, cited from the literature, showing: simultaneous formation of multiple centrioles and basal bodies; cytoplasmic synthesis of thymidine triphosphate; presence in the cytoplasm of particles capable of performing RNAinstructed synthesis of DNA; presence of reverse transcriptase in eukaryotic cells not infected with RNA viruses; formation of centrioles in homogenates of artificially activated surf clam eggs. The relationship of this hypothesis to the postulated generative mechanism for centriole duplication is also discussed. The concept of reproductive continuity in centrioles is an old one, dating back at least to Boveri (1900). The earlier evidence to support the concept of reproductive continuity in centrioles was morphological, based first upon light microscope observations and later on electron microscope observations (Went, 1966). The discovery of DNA in mitochondria and chloroplasts led to the tentative correlation of DNA with all cell organelles exhibiting reproductive continuity. Attempts to demonstrate the presence of DNA in centrioles were not convincing, yet the data were not clearly negative. A re-evaluation of some of the earlier data (Hartman, 1975) along with a consideration of newer data (Dippell, 1976; Hartman, Puma & Gurney, 1974; Stubblefield & Brinkley, 1967) appears to support more strongly the idea that RNA exists in centrioles and basal bodies rather than DNA. Structures exhibiting reproductive continuity (mitochondria, chloroplasts, viruses) include a molecular replication phase of endogenous DNA or RNA 95
96
H.
A.
WENT
in their duplication cycle. It is very appealing to consider that a simih process (i.e. molecular replication) occurs in association with centriole duplication. This has already been postulated (Mazia, 1961). DNA had been envisaged by some (including the author) to be the primary genome of the centriole and it was thought to be replicated during the period of centriole duplication. When it appeared that the nucleic acid detected in centrioles (and basal bodies) was more likely to be RNA, the interesting suggestion was made (Hartman, 1975) that the centriolar RNA was synthesized along a nuclear DNA template. This hypothesis places the primary genome for the centriole in the nucleus which implies that a cell and its descendants retain the genetic capacity to form centrioles as long as the nuclear genome of the cell remains intact, whether or not there are any centrioles and procentrioles present. It has also the appeal of reconciling the apparent absence of DNA from the organelle with the molecular replication concept. However, on the basis of published information, a possible objection to Hartman’s hypothesis comes to mind. It requires the transcription of the centriolar RNA along the nuclear DNA template to occur during the S phase of the cell cycle since it appears that periods of centriole duplication coincide, at least partially, with S phases (Alfert, 1950; Hinegardner, Rao & Feldman, 1964; Pasteels & Lison, 1951; Robbins, Jentzsch & Micali, 1968; Simmel & Karnofsky, 1961; Swift & Kleinfeld, 1953). It has already been suggested by Mazia (1961) that centriole duplication follows a generative rather than a fission mechanism. The generative mechanism postulates the existence of three distinct phases in the centriole duplication cycle: (1) the generative phase during which a “seed” (or procentriole is formed); (2) the growth phase during which the “seed” (or procentriole) develops into or directs the development of the mature centriole, or serves as a nucleating center for a specific pattern of microtubules, (3) the separation phase during which the parent and daughter centrioles migrate away from each other. The separation and growth phases will not concern us here. The main concern of this paper is with the generative phase. Mazia (1961) has already suggested that this phase of centriole duplication involves a molecular replication. This implicates DNA and/or RNA and is thought to be the means whereby reproductive continuity can be maintained through successive generations of centrioles. Since DNA and RNA are the only molecules known to replicate themselves, two questions arise. Are either or both involved and what is the sequence of steps followed? Accepting tentatively that centrioles and basal bodies contain RNA, I would like to make the alternative suggestion that this RNA is the primary genome of the organelle, coding for at least some centriolar proteins, and that it is replicated via a DNA intermediate with the aid of a reverse trans-
CENTRIOLE
DUPLICATION
97
criptase (RNA-instructed DNA poIymerase) into more RNA. This suggestion is presented only as a new alternative compatible with existing data and is not intended to replace any extant hypotheses. In keeping with this, the data discussed below in support of this alternative are also consistent with other interpretations. The following six reasons (not necessarily listed in order of importance) are cited in support of this new alternative. (1) It would permit all events associated with centriole duplication to occur in the cytoplasm and hence in the immediate vicinity of the parent structure. It has been demonstrated by Gallo, Miller, Saxinger & Gillespie (1973) that cytoplasmic particles from leucocytes of humans with acute myeloblastic leukemia can synthesize DNA by use of an RNA primer and template. Other important observations bearing on whether or not the generative phase can be restricted entirely to the cytoplasm were reported by Nishioka & Mazia (1976). They observed the synthesis of thymidine triphosphate, a substrate for both nuclear DNA polymerase and reverse transcriptase, by ammonia-treated enucleate fragments of sea urchin eggs. Of equal importance, their data also show that the synthesis of thymidine triphosphate is essentially restricted to the S phase in both fertilized and ammonia-treated unfertilized eggs. In these nucleated cells the thymidine triphosphate does not accumulate because its rate of synthesis does not exceed the pace at which it is utilized in DNA synthesis. (2) It provides another role for the enzymes with reverse transcriptase properties which have been detected in some eukaryotic cells not infected by RNA viruses (Bobrow, Smith, Reitz & Gallo, 1972; Coffin & Temin, 1971; Kang & Temin, 1972). Only in Xenopus oijcytes has a specific role been demonstrated for the reverse transcriptase which had been detected. In this case it appears to be involved in gene amplification resulting in the production of many copies of ribosomaf DNA (Brown & Tocchini-Valentini, 1974; Ficq & Brachet, 1971; Mahdavi & Crippa, 1972). However, other investigators suggest that ribosomal gene amplification in Xenopus occurs by rolling circles of DNA (Bird, 1974; Rochaix & Bird, 1975; Rochaix, Bird & Bakken, 1974) without the intervention of transcriptase. There are other reports of ribosomal DNA amplification in oocytes (Cave, 1972; Dawid & Brown, 1970; Ullman, Lima-de-Faria, Jaworska Sr. Bryngelsson, 1973). however these do not mention reverse transcriptase. (3) It would permit reproductive continuity in the absence of morphological continuity demonstrable by electron microscopy (Dirksen, 197 1; Kalnins & Porter, 1969; Sorokin, 1968; Steinman, 1968). (4) It is consistent with the nearly simultaneous formation of multiple centrioles and/or basal bodies from amorphous granular or fibrogranular regions observed in the cells of differentiating ciliated epithelium (Dirksen, I.“. 7
98
H.
A.
WENT
1971; Kalnins & Porter, 1969; Sorokin, 1968; Steinman, 1968). At the time that the new centrioles or basal bodies are developing the structureless granular or fibrogranular regions appear to bear no direct morphological relationship to the parent centrioles. These cases clearly cannot be the result of a procentriole developing into a mature centriole before another procentriole can be formed (typical of mitotic centrioles), because then one would see an exponential increase in mature centrioles (or basal bodies) instead of the essentially none-to-all increase observed. The development of multiple centrioles from diffuse perinuclear material has been reported (PickettHeaps, 1971), but in this case the photographs can easily be interpreted as showing new centrioles developing one at a time at each end of the growing line of individual centrioles. In oitro formation of microtubules into asters containing centrioles has been observed to occur in homogenates of artificially activated surf clam eggs (Weisenberg & Rosenfeld, 1975). These homogenates were obtained from cells which would not be expected to contain mature centrioles, therefore the centrioles that were detected developed in the absence of morphological continuity with any parent centrioles; a situation reminiscent of the observations on multiple basal body formation in differentiating ciliated epithelium. No asters were seen to develop in homogenates from unactivated eggs. Asters developing in homogenates obtained immediately after activation did not contain clearly identifiable centrioles, while the asters which developed in homogenates obtained 4.5 min after activation were organized around distinct centrioles. The authors did not indicate the number of centrioles detectable per egg used in the homogenate. These data suggest that there were no mature centrioles in the eggs before activation, but that procentrioles or “seeds” were present. This same explanation can also account for the presence of centrioles in cytasters that were induced in artificially activated unfertilized sea urchin eggs. (5) It would mean that the RNA present in the organelle represents the primary genome of the structure rather than a transcript complementary to the primary genome located elsewhere in the cell, such as the nucleus (Hartman, 1975). This is consistent with other structures exhibiting reproductive continuity (mitochondria, chloroplasts, viruses). (6) It is consistent with some of the author’s unpublished data which strongly implicate active DNA synthesis in centriole duplication in sand dollar eggs. The postulated mechanism involving reverse transcriptase in the duplication cycle is as follows. The centriolar RNA is transcribed into DNA to form an RNA-DNA duplex under the influence of an RNA-instructed DNA polymerase. The DNA will then dissociate itself from the RNA template and can be retranscribed into more centriolar RNA identical to the original molecule.
CENTRIOLE
DUPLICATION
99
This sequence will be considered the generative phase. During one of these steps amplification can occur to lead to the formation of multiple procentrioles necessary for the simultaneous formation of many basal bodies or centrioles observed in certain types of cells. This amplification is, of course, not required when only a doubling in the number of centrioles during each cell cycle is adequate to provide enough for mitosis. It is suggested that these events of the generative phase occur in the cytoplasm within the immediate vicinity of the parent centriole. Therefore, it represents a departure from an accepted hypothesis for the multiplication of RNA viruses that have their RNA replicated via a DNA intermediate, for in these cases the intermediate DNA transcript becomes double stranded and is inserted into a nuclear genophore before retranscription into viral RNA occurs (Bishop & Flamand, 1975). The idea developed in this paper is based on a statement to the author by Dr Howard Hosick, who is a member of the author’s department. The author also sincerely acknowledges the valuable suggestions of Professor Daniel Mazia made while this paper was in manuscript form. REFERENCES ALFERT, M. (1950). J. cell. camp. BIRD, A. P. (1974). Chromosoma BISHOP, D. H. L. & FLAMAND,
Physiol. 36. 381. 46,421. A. (1975).In Control Processes in Virus MuItiplication
(D. C. Burke & W. C. Russel,eds),25thSymposium of the Societyfor GeneralMicrobiology, p. 95.Cambridge:CambridgeUniversityPress. BOBROW, S. N., SMITH, R. G., REITZ, M. S. & GALLO, R. C. (1972).Proc. natn. Acad. Sci. U.S.A. 69, 3228. TH. (1900).7%er die Nutrtr der Centrosomen. Zellen-Studien 4, p. 1. Jena:Gustav Fischer. BROWN, R. D. & TOCCHINI-VALENTINI, G. P. (1974). Meth. Enzynrol. 29. part E, 1973. CAVE, M. D. (1972). J. Cell Biol. 55, 310. COFFIN, J. M. & TEMIN, H. M. (1971).J. Viral. 8, 630. DAWID, I. B. & BROWN, D. ID. (1970).Devl Bin/. 22, I. BOVERI,
DIPPELL, R. V. (1976). J. Ceft Biol. 69, 622. DIRKSEN, E. (1971). J. Cell Eiol. 51, 286. FICQ, A. & BRACHET, J. (1971). Proc. natn. Acad. Sci. U.S.A. 68,2774. GALLO, R. C., MILLER, N. R., SAXINGER, W. C. & GILLESPIE, D. (1973). Proc. rzafn. Am/. Sri. U.S.A. 70, 3219. HARTMAN, H. (1975). J. theor. Biol. 51, 501. HARTMAN, H., PUMA, J. & GURNEY, T. (1974). J. Cell Sci. 16, 241. HINEGARDNER, R. T., RAO, B. & FELDMAN, D. E. (1964).Expl Cell Res. 36, 53. KALNINS, V. I. & PORTER, K. R. (1969). Z. zellforsch. mikrosk. Anat. 100, I. KANG, C. & TEMIN, H. M. (1972). Proc. natn. Acad. Sri. U.S.A. 69, 1550. MAHDAVI, V. & CRIPPA, M. (1972). Proc. natn. Acad. Sci. U.S.A. 69, 1749. MAZIA, D. (1961). In The Ceil, Vol. 3, p. 77 (J. Brachet & A. E. Mirsky, eds). New York: Academic Press. NISHIOKA, D. & MAZIA, D. (1977). in press. PASTEELS, J. & LWN, L. (1951). Nature, Lond. 167, 948.
100
I$.
A.
WENT
PICKETT-HEAPS, J. (1971). Pro~oplus~u~ 72, 275. ROBBINS, E., JENTZSCH, G. & MICALI, A. (1968). J. Cell Biol. 36, 329. ROCHAIX. J. D. & BIRD. A. P. (1975l Chromosoma 52. 317. ROCHAIX; J. D., BIRD, A. P. &‘BAK~EN, A. (1974). J. molec. Biol. 87, 473. SIMMEL, E. B. & KARNOFSKY, D. A. (1961). J. &o&s. biochem. Cytol. 10, 59. SOROKIN, S. (1968). J. Cell Sci. 3, 207. STEINMAN, R. M. (1968). An?. J. Anar. 122, 19. STUBBLEFIELD, E. & BRINKLEY, B. R. (1967). In Formation and Fate of Cell p. 175 (K. B. Warren, ed.). New York: Academic Press. SWIM, H. & KLEINFELD, R. (1953). PhysioI. Zoiil. 26, 301. ULLMAN, D. S., LIMA-DE-FARIA, A., JAWORSKA, H. & BRYNOELSSON, T. (1973). 74, 13. WEISENBERG, R. C. & ROSENFELD, A. C. (1975). J. Cell Biol. 64, 146. WENT, H. A. (1966). Protoplasmatologiu band VI, G, 1.
Organelles, Hereditas
Can a Reverse Transcriptuse be Involved in Centriole Duplication? HANS A. WENT Department of Zoology, Washington State University, Pullman, Washington 99164, U.S.A. (Received 2 November 1976, and in revisedform
18 February
1977)
Accepting tentatively that the nucleic acid thought to be present in centrioles and basal bodies is RNA rather than DNA, which had earlier been thought to be the case, it is suggested that this RNA represents the primary genome of the organelle which is replicated via a DNA intermediate with the aid of an RNA-instructed DNA polymerase (reverse transcriptase) into more RNA during the course of the duplication cycle. This suggestion is presented as an alternate, and not as a replacement, to one made earlier which states that the organellar RNA is a transcript of the primary DNA genome located in the nucleus. In support of this hypothesis are data, cited from the literature, showing: simultaneous formation of multiple centrioles and basal bodies; cytoplasmic synthesis of thymidine triphosphate; presence in the cytoplasm of particles capable of performing RNAinstructed synthesis of DNA; presence of reverse transcriptase in eukaryotic cells not infected with RNA viruses; formation of centrioles in homogenates of artificially activated surf clam eggs. The relationship of this hypothesis to the postulated generative mechanism for centriole duplication is also discussed. The concept of reproductive continuity in centrioles is an old one, dating back at least to Boveri (1900). The earlier evidence to support the concept of reproductive continuity in centrioles was morphological, based first upon light microscope observations and later on electron microscope observations (Went, 1966). The discovery of DNA in mitochondria and chloroplasts led to the tentative correlation of DNA with all cell organelles exhibiting reproductive continuity. Attempts to demonstrate the presence of DNA in centrioles were not convincing, yet the data were not clearly negative. A re-evaluation of some of the earlier data (Hartman, 1975) along with a consideration of newer data (Dippell, 1976; Hartman, Puma & Gurney, 1974; Stubblefield & Brinkley, 1967) appears to support more strongly the idea that RNA exists in centrioles and basal bodies rather than DNA. Structures exhibiting reproductive continuity (mitochondria, chloroplasts, viruses) include a molecular replication phase of endogenous DNA or RNA 95
96
H.
A.
WENT
in their duplication cycle. It is very appealing to consider that a simih process (i.e. molecular replication) occurs in association with centriole duplication. This has already been postulated (Mazia, 1961). DNA had been envisaged by some (including the author) to be the primary genome of the centriole and it was thought to be replicated during the period of centriole duplication. When it appeared that the nucleic acid detected in centrioles (and basal bodies) was more likely to be RNA, the interesting suggestion was made (Hartman, 1975) that the centriolar RNA was synthesized along a nuclear DNA template. This hypothesis places the primary genome for the centriole in the nucleus which implies that a cell and its descendants retain the genetic capacity to form centrioles as long as the nuclear genome of the cell remains intact, whether or not there are any centrioles and procentrioles present. It has also the appeal of reconciling the apparent absence of DNA from the organelle with the molecular replication concept. However, on the basis of published information, a possible objection to Hartman’s hypothesis comes to mind. It requires the transcription of the centriolar RNA along the nuclear DNA template to occur during the S phase of the cell cycle since it appears that periods of centriole duplication coincide, at least partially, with S phases (Alfert, 1950; Hinegardner, Rao & Feldman, 1964; Pasteels & Lison, 1951; Robbins, Jentzsch & Micali, 1968; Simmel & Karnofsky, 1961; Swift & Kleinfeld, 1953). It has already been suggested by Mazia (1961) that centriole duplication follows a generative rather than a fission mechanism. The generative mechanism postulates the existence of three distinct phases in the centriole duplication cycle: (1) the generative phase during which a “seed” (or procentriole is formed); (2) the growth phase during which the “seed” (or procentriole) develops into or directs the development of the mature centriole, or serves as a nucleating center for a specific pattern of microtubules, (3) the separation phase during which the parent and daughter centrioles migrate away from each other. The separation and growth phases will not concern us here. The main concern of this paper is with the generative phase. Mazia (1961) has already suggested that this phase of centriole duplication involves a molecular replication. This implicates DNA and/or RNA and is thought to be the means whereby reproductive continuity can be maintained through successive generations of centrioles. Since DNA and RNA are the only molecules known to replicate themselves, two questions arise. Are either or both involved and what is the sequence of steps followed? Accepting tentatively that centrioles and basal bodies contain RNA, I would like to make the alternative suggestion that this RNA is the primary genome of the organelle, coding for at least some centriolar proteins, and that it is replicated via a DNA intermediate with the aid of a reverse trans-
CENTRIOLE
DUPLICATION
97
criptase (RNA-instructed DNA poIymerase) into more RNA. This suggestion is presented only as a new alternative compatible with existing data and is not intended to replace any extant hypotheses. In keeping with this, the data discussed below in support of this alternative are also consistent with other interpretations. The following six reasons (not necessarily listed in order of importance) are cited in support of this new alternative. (1) It would permit all events associated with centriole duplication to occur in the cytoplasm and hence in the immediate vicinity of the parent structure. It has been demonstrated by Gallo, Miller, Saxinger & Gillespie (1973) that cytoplasmic particles from leucocytes of humans with acute myeloblastic leukemia can synthesize DNA by use of an RNA primer and template. Other important observations bearing on whether or not the generative phase can be restricted entirely to the cytoplasm were reported by Nishioka & Mazia (1976). They observed the synthesis of thymidine triphosphate, a substrate for both nuclear DNA polymerase and reverse transcriptase, by ammonia-treated enucleate fragments of sea urchin eggs. Of equal importance, their data also show that the synthesis of thymidine triphosphate is essentially restricted to the S phase in both fertilized and ammonia-treated unfertilized eggs. In these nucleated cells the thymidine triphosphate does not accumulate because its rate of synthesis does not exceed the pace at which it is utilized in DNA synthesis. (2) It provides another role for the enzymes with reverse transcriptase properties which have been detected in some eukaryotic cells not infected by RNA viruses (Bobrow, Smith, Reitz & Gallo, 1972; Coffin & Temin, 1971; Kang & Temin, 1972). Only in Xenopus oijcytes has a specific role been demonstrated for the reverse transcriptase which had been detected. In this case it appears to be involved in gene amplification resulting in the production of many copies of ribosomaf DNA (Brown & Tocchini-Valentini, 1974; Ficq & Brachet, 1971; Mahdavi & Crippa, 1972). However, other investigators suggest that ribosomal gene amplification in Xenopus occurs by rolling circles of DNA (Bird, 1974; Rochaix & Bird, 1975; Rochaix, Bird & Bakken, 1974) without the intervention of transcriptase. There are other reports of ribosomal DNA amplification in oocytes (Cave, 1972; Dawid & Brown, 1970; Ullman, Lima-de-Faria, Jaworska Sr. Bryngelsson, 1973). however these do not mention reverse transcriptase. (3) It would permit reproductive continuity in the absence of morphological continuity demonstrable by electron microscopy (Dirksen, 197 1; Kalnins & Porter, 1969; Sorokin, 1968; Steinman, 1968). (4) It is consistent with the nearly simultaneous formation of multiple centrioles and/or basal bodies from amorphous granular or fibrogranular regions observed in the cells of differentiating ciliated epithelium (Dirksen, I.“. 7
98
H.
A.
WENT
1971; Kalnins & Porter, 1969; Sorokin, 1968; Steinman, 1968). At the time that the new centrioles or basal bodies are developing the structureless granular or fibrogranular regions appear to bear no direct morphological relationship to the parent centrioles. These cases clearly cannot be the result of a procentriole developing into a mature centriole before another procentriole can be formed (typical of mitotic centrioles), because then one would see an exponential increase in mature centrioles (or basal bodies) instead of the essentially none-to-all increase observed. The development of multiple centrioles from diffuse perinuclear material has been reported (PickettHeaps, 1971), but in this case the photographs can easily be interpreted as showing new centrioles developing one at a time at each end of the growing line of individual centrioles. In oitro formation of microtubules into asters containing centrioles has been observed to occur in homogenates of artificially activated surf clam eggs (Weisenberg & Rosenfeld, 1975). These homogenates were obtained from cells which would not be expected to contain mature centrioles, therefore the centrioles that were detected developed in the absence of morphological continuity with any parent centrioles; a situation reminiscent of the observations on multiple basal body formation in differentiating ciliated epithelium. No asters were seen to develop in homogenates from unactivated eggs. Asters developing in homogenates obtained immediately after activation did not contain clearly identifiable centrioles, while the asters which developed in homogenates obtained 4.5 min after activation were organized around distinct centrioles. The authors did not indicate the number of centrioles detectable per egg used in the homogenate. These data suggest that there were no mature centrioles in the eggs before activation, but that procentrioles or “seeds” were present. This same explanation can also account for the presence of centrioles in cytasters that were induced in artificially activated unfertilized sea urchin eggs. (5) It would mean that the RNA present in the organelle represents the primary genome of the structure rather than a transcript complementary to the primary genome located elsewhere in the cell, such as the nucleus (Hartman, 1975). This is consistent with other structures exhibiting reproductive continuity (mitochondria, chloroplasts, viruses). (6) It is consistent with some of the author’s unpublished data which strongly implicate active DNA synthesis in centriole duplication in sand dollar eggs. The postulated mechanism involving reverse transcriptase in the duplication cycle is as follows. The centriolar RNA is transcribed into DNA to form an RNA-DNA duplex under the influence of an RNA-instructed DNA polymerase. The DNA will then dissociate itself from the RNA template and can be retranscribed into more centriolar RNA identical to the original molecule.
CENTRIOLE
DUPLICATION
99
This sequence will be considered the generative phase. During one of these steps amplification can occur to lead to the formation of multiple procentrioles necessary for the simultaneous formation of many basal bodies or centrioles observed in certain types of cells. This amplification is, of course, not required when only a doubling in the number of centrioles during each cell cycle is adequate to provide enough for mitosis. It is suggested that these events of the generative phase occur in the cytoplasm within the immediate vicinity of the parent centriole. Therefore, it represents a departure from an accepted hypothesis for the multiplication of RNA viruses that have their RNA replicated via a DNA intermediate, for in these cases the intermediate DNA transcript becomes double stranded and is inserted into a nuclear genophore before retranscription into viral RNA occurs (Bishop & Flamand, 1975). The idea developed in this paper is based on a statement to the author by Dr Howard Hosick, who is a member of the author’s department. The author also sincerely acknowledges the valuable suggestions of Professor Daniel Mazia made while this paper was in manuscript form. REFERENCES ALFERT, M. (1950). J. cell. camp. BIRD, A. P. (1974). Chromosoma BISHOP, D. H. L. & FLAMAND,
Physiol. 36. 381. 46,421. A. (1975).In Control Processes in Virus MuItiplication
(D. C. Burke & W. C. Russel,eds),25thSymposium of the Societyfor GeneralMicrobiology, p. 95.Cambridge:CambridgeUniversityPress. BOBROW, S. N., SMITH, R. G., REITZ, M. S. & GALLO, R. C. (1972).Proc. natn. Acad. Sci. U.S.A. 69, 3228. TH. (1900).7%er die Nutrtr der Centrosomen. Zellen-Studien 4, p. 1. Jena:Gustav Fischer. BROWN, R. D. & TOCCHINI-VALENTINI, G. P. (1974). Meth. Enzynrol. 29. part E, 1973. CAVE, M. D. (1972). J. Cell Biol. 55, 310. COFFIN, J. M. & TEMIN, H. M. (1971).J. Viral. 8, 630. DAWID, I. B. & BROWN, D. ID. (1970).Devl Bin/. 22, I. BOVERI,
DIPPELL, R. V. (1976). J. Ceft Biol. 69, 622. DIRKSEN, E. (1971). J. Cell Eiol. 51, 286. FICQ, A. & BRACHET, J. (1971). Proc. natn. Acad. Sci. U.S.A. 68,2774. GALLO, R. C., MILLER, N. R., SAXINGER, W. C. & GILLESPIE, D. (1973). Proc. rzafn. Am/. Sri. U.S.A. 70, 3219. HARTMAN, H. (1975). J. theor. Biol. 51, 501. HARTMAN, H., PUMA, J. & GURNEY, T. (1974). J. Cell Sci. 16, 241. HINEGARDNER, R. T., RAO, B. & FELDMAN, D. E. (1964).Expl Cell Res. 36, 53. KALNINS, V. I. & PORTER, K. R. (1969). Z. zellforsch. mikrosk. Anat. 100, I. KANG, C. & TEMIN, H. M. (1972). Proc. natn. Acad. Sri. U.S.A. 69, 1550. MAHDAVI, V. & CRIPPA, M. (1972). Proc. natn. Acad. Sci. U.S.A. 69, 1749. MAZIA, D. (1961). In The Ceil, Vol. 3, p. 77 (J. Brachet & A. E. Mirsky, eds). New York: Academic Press. NISHIOKA, D. & MAZIA, D. (1977). in press. PASTEELS, J. & LWN, L. (1951). Nature, Lond. 167, 948.
100
I$.
A.
WENT
PICKETT-HEAPS, J. (1971). Pro~oplus~u~ 72, 275. ROBBINS, E., JENTZSCH, G. & MICALI, A. (1968). J. Cell Biol. 36, 329. ROCHAIX. J. D. & BIRD. A. P. (1975l Chromosoma 52. 317. ROCHAIX; J. D., BIRD, A. P. &‘BAK~EN, A. (1974). J. molec. Biol. 87, 473. SIMMEL, E. B. & KARNOFSKY, D. A. (1961). J. &o&s. biochem. Cytol. 10, 59. SOROKIN, S. (1968). J. Cell Sci. 3, 207. STEINMAN, R. M. (1968). An?. J. Anar. 122, 19. STUBBLEFIELD, E. & BRINKLEY, B. R. (1967). In Formation and Fate of Cell p. 175 (K. B. Warren, ed.). New York: Academic Press. SWIM, H. & KLEINFELD, R. (1953). PhysioI. Zoiil. 26, 301. ULLMAN, D. S., LIMA-DE-FARIA, A., JAWORSKA, H. & BRYNOELSSON, T. (1973). 74, 13. WEISENBERG, R. C. & ROSENFELD, A. C. (1975). J. Cell Biol. 64, 146. WENT, H. A. (1966). Protoplasmatologiu band VI, G, 1.
Organelles, Hereditas
