Submitted to: American Society for Microbiology
Publication Type: Abstract only
Publication Acceptance Date: 4/20/2006
Publication Date: 5/15/2006
Citation: Loveless, T.M., Mylona, P.V., Ricke, S.C., Bishop, P.E. 2006. An abc transport-like system in azotobacter vinelandii involved in nitrogenase regulation and molybdenum sensing . American Society for Microbiology. Interpretive Summary:
Technical Abstract: Azotobacter vinelandii contains three genetically distinct nitrogenases which are expressed in response to the presence or absence of molybdenum (Mo) or vanadium (V) in N-free growth medium. Mo represses expression of the two Mo-independent nitrogenases. Tungsten (W) inhibits diazotrophic growth of A. vinelandii by inhibition of Mo uptake and by its inability to substitute for Mo in catalytically active Mo-dependent nitrogenase 1. However, a spontaneous W-tolerant mutant of A. vinelandii (strain CA6) constitutively expresses Mo-independent nitrogenase 3 in the presence of normally repressive concentrations of Mo or tungsten. We isolated a l.7-kbp SalI-SphI fragment from CA6 genomic DNA that contains mutations responsible for the W-tolerance phenotype of CA6. Three genes identified in the draft genomic sequence of A. vinelandii were found to correspond to the mutated region in CA6 genomic DNA. We isolated and designated these genes as mohC, mohA, and mohB. Their deduced amino acid sequence shows extensive homology with peripheral proteins of the cytoplasmic membrane which are components of bacterial ABC (ATP-binding cassette) transport systems. Mutants with kan interposon insertions in genes mohC, mohA and mohB were designated as CA125, CA130, and CA129 respectively. All three strains exhibit a W-tolerance phenotype similar to that observed for CA6. Molybdenum accumulation studies indicate that Mo-uptake is impaired in both the MohC- (ATP-binding protein) and MohB- (membrane protein) but not in MohA- (periplasmic substrate-binding protein) mutants. Expression from a mohC-lacZ transcriptional fusion shows that after a 20-h period of derepression, beta-galactosidase activity increased two-fold in the presence of 100 mM sodium molybdate. Competition experiments indicate that wild-type has an advantage over CA6 under N2-fixing conditions in the presence or absence of Mo. In conclusion, the results of this study suggest that mohC, mohA and mohB are involved in sensing and uptake of Mo, and regulation of nitrogenases by Mo.