0021-9193/78/0133-0415$02.00/0 JOURNAL OF BACTERIOLOGY, Jan. 1978, p. 415-417 Copyright 1978 American Society for Microbiology
Vol. 133, No. 1
Printed in U.S.A.
Interaction Between the Fumarate Reductase System of Escherichia coli and the Nitrogen Fixation Genes of Klebsiella pneumoniae MARY L. SKOTNICKI AND BARRY G. ROLFE* Genetics Department, Research School ofBiological Sciences, The Australian National University, Canberra, A.C.T. 2601, Australia Received for publication 4 May 1977
For phenotypic expression of niflp genes in Escherichia coli K-12, the anaerobic electron transport system to fumarate must be functional. The role of the fumarate reduction system is to energize the membrane and thus provide the energy necessary for nitrogen fixation.
Nitrogen fixation depends on appropriate electron transport reactions, on adequate energy supplies, and probably on the incorporation of the nitrogenase enzyme into an organizational unit associated with membranes (7). Because of the availability of genetically and biochemically well-characterized mutants in Escherichia coli, we have constructed hybrid strains carrying an F-prime niftp plasmid to study which energy pathways might be coupled to nitrogenase activity for nitrogen fixation in E. coli K-12. Anaerobically, the membrane may be energized either by coupling to available ATP, the utilization of which requires a coupled intact Mg2e-stimulated adenosine triphosphatase (ATPase) (4), or to the electron flow to the acceptor nitrate or fumarate (6). It was previously shown (12) that the Nif+ phenotype still occurred when the Mg,-stimulated ATPase activity was abolished by mutation. As it was also known that nitrate inhibits nitrogenase activity (13), the role of the fumarate reductase system in nitrogen fixation was investigated by using E. coli K-12 (F'niftp) hybrids. When the FN68 plasmid (F'niftlp Cbr) was transferred into well-defined mutants of E. coli K-12, selecting for transfer of carbenicillin resistance (11, 12), two classes of hybrids were found. Class I hybrids have a defective Nif phenotype; they grow slowly on nitrogen-free medium and reduce acetylene at a very slow rate of about 0.1 to 0.2% of that found with the isogenic parental hybrid strain. Class II hybrids, however, are phenotypically Nif+ and reduce acetylene at rates comparable to those of Klebsiella pneumoniae M5a1, the bacterium from which the nifKp genes were derived. E. coli K-12 with a nonfunctional fumarate reductase (frd) forms a hybrid, AN472(FN68), which has a Nif-defective phenotype (class I).
This response cannot be altered by the addition of fumarate or succinate (Table 1). The Nif+ phenotype of the class II uncB hybrid uncoupled in oxidative phosphorylation, AN283(FN68) (2), was abolished in strain AN480 in which both mutations (frd and uncB) were present; hybrid AN480(FN68) showed a class I response. Moreover, the phenotypic expression of nifiKp genes in E. coli K-12 is dependent on the presence of menaquinone (involved in anaerobic electron transport to fmarate [3, 9, 10]) as illustrated by the class I response of the menA hybrid AN386(FN68) and the menA ubiA hybrid AN384(FN68) (Table 1). Ubiquinone is not necessary for the Nif+ phenotype, since the ubiA hybrid AN385(FN68) showed a normal level of acetylene reduction. When the slightly leaky menA hybrid AN99(FN68) was tested, a low level of acetylene reduction was detected. This nitrogenase activity was stimulated sixfold by the addition of 20 mM fumarate, suggesting that menaquinone may play a role in fumarate biosynthesis as well as in its reduction. Succinate addition was also tested in these experiments to check whether the important role played by fumarate in nitrogen fixation was to supply succinate. The results show that stimulation of acetylene reduction is specific for fumarate and not succinate (Table 1) and indicate a requirement for an anaerobic electron transport system involving fumarate for expression of the Nif' phenotype in E. coli K12. It was possible to transfer the nifKp genes on plasmid FN68 from class I hybrids back to the original host strain SB1801 with full expression, demonstrating that the class I Nif-defective phenotype could not have resulted by segregation of the nifZp genes from plasmid FN68. To test whether the anaerobic electron trans415
416
J. BACTERIOL
NOTES
TABLE 1. Phenotypic expression of nifk,, genes carried on plasmid FN68 in ubi, men, and frd, mutants of E. coli K-12a Acetylene reductionb
Hybrid
Relevant
otype
gen-
TABLE 2. Effect of addition of uncoupler CCCP on the Nif phenotype of hybrid strausa Strain and relevant genotype
Addition
NFM + NFM + NFMc
20mM
20mM
AN259(FN68) AN472(FN68) AN283(FN68) AN480(FN68)
unc+ frd+ frd uncB frd uncB
fumarate succinate 42.2 36.9 36.5
Vol. 133, No. 1
Printed in U.S.A.
Interaction Between the Fumarate Reductase System of Escherichia coli and the Nitrogen Fixation Genes of Klebsiella pneumoniae MARY L. SKOTNICKI AND BARRY G. ROLFE* Genetics Department, Research School ofBiological Sciences, The Australian National University, Canberra, A.C.T. 2601, Australia Received for publication 4 May 1977
For phenotypic expression of niflp genes in Escherichia coli K-12, the anaerobic electron transport system to fumarate must be functional. The role of the fumarate reduction system is to energize the membrane and thus provide the energy necessary for nitrogen fixation.
Nitrogen fixation depends on appropriate electron transport reactions, on adequate energy supplies, and probably on the incorporation of the nitrogenase enzyme into an organizational unit associated with membranes (7). Because of the availability of genetically and biochemically well-characterized mutants in Escherichia coli, we have constructed hybrid strains carrying an F-prime niftp plasmid to study which energy pathways might be coupled to nitrogenase activity for nitrogen fixation in E. coli K-12. Anaerobically, the membrane may be energized either by coupling to available ATP, the utilization of which requires a coupled intact Mg2e-stimulated adenosine triphosphatase (ATPase) (4), or to the electron flow to the acceptor nitrate or fumarate (6). It was previously shown (12) that the Nif+ phenotype still occurred when the Mg,-stimulated ATPase activity was abolished by mutation. As it was also known that nitrate inhibits nitrogenase activity (13), the role of the fumarate reductase system in nitrogen fixation was investigated by using E. coli K-12 (F'niftp) hybrids. When the FN68 plasmid (F'niftlp Cbr) was transferred into well-defined mutants of E. coli K-12, selecting for transfer of carbenicillin resistance (11, 12), two classes of hybrids were found. Class I hybrids have a defective Nif phenotype; they grow slowly on nitrogen-free medium and reduce acetylene at a very slow rate of about 0.1 to 0.2% of that found with the isogenic parental hybrid strain. Class II hybrids, however, are phenotypically Nif+ and reduce acetylene at rates comparable to those of Klebsiella pneumoniae M5a1, the bacterium from which the nifKp genes were derived. E. coli K-12 with a nonfunctional fumarate reductase (frd) forms a hybrid, AN472(FN68), which has a Nif-defective phenotype (class I).
This response cannot be altered by the addition of fumarate or succinate (Table 1). The Nif+ phenotype of the class II uncB hybrid uncoupled in oxidative phosphorylation, AN283(FN68) (2), was abolished in strain AN480 in which both mutations (frd and uncB) were present; hybrid AN480(FN68) showed a class I response. Moreover, the phenotypic expression of nifiKp genes in E. coli K-12 is dependent on the presence of menaquinone (involved in anaerobic electron transport to fmarate [3, 9, 10]) as illustrated by the class I response of the menA hybrid AN386(FN68) and the menA ubiA hybrid AN384(FN68) (Table 1). Ubiquinone is not necessary for the Nif+ phenotype, since the ubiA hybrid AN385(FN68) showed a normal level of acetylene reduction. When the slightly leaky menA hybrid AN99(FN68) was tested, a low level of acetylene reduction was detected. This nitrogenase activity was stimulated sixfold by the addition of 20 mM fumarate, suggesting that menaquinone may play a role in fumarate biosynthesis as well as in its reduction. Succinate addition was also tested in these experiments to check whether the important role played by fumarate in nitrogen fixation was to supply succinate. The results show that stimulation of acetylene reduction is specific for fumarate and not succinate (Table 1) and indicate a requirement for an anaerobic electron transport system involving fumarate for expression of the Nif' phenotype in E. coli K12. It was possible to transfer the nifKp genes on plasmid FN68 from class I hybrids back to the original host strain SB1801 with full expression, demonstrating that the class I Nif-defective phenotype could not have resulted by segregation of the nifZp genes from plasmid FN68. To test whether the anaerobic electron trans415
416
J. BACTERIOL
NOTES
TABLE 1. Phenotypic expression of nifk,, genes carried on plasmid FN68 in ubi, men, and frd, mutants of E. coli K-12a Acetylene reductionb
Hybrid
Relevant
otype
gen-
TABLE 2. Effect of addition of uncoupler CCCP on the Nif phenotype of hybrid strausa Strain and relevant genotype
Addition
NFM + NFM + NFMc
20mM
20mM
AN259(FN68) AN472(FN68) AN283(FN68) AN480(FN68)
unc+ frd+ frd uncB frd uncB
fumarate succinate 42.2 36.9 36.5
