RESUMO
Nucleotide sequence was obtained for a region of 7,099 bp spanning the nifU, nifS, nifV, nifW, nifZ, and nifM genes from Azotobacter chroococcum. Chromosomal mutations constructed at several sites within the locus confirmed a requirement for this region for expression of the molybdenum nitrogenase in this organism. The genes are tightly clustered and ordered as in Klebsiella pneumoniae except for two additional open reading frames (ORFs) between nifV and nifW. The arrangement of genes in A. chroococcum closely matches that described for Azotobacter vinelandii. The polypeptide encoded by ORF4 immediately downstream from nifV is 41% identical over 186 amino acids to the product of the cysE gene from Escherichia coli, which encodes serine acetyltransferase (SAT), a key enzyme in cysteine biosynthesis. Plasmids which potentially express ORF4 complemented E. coli JM39, a cysteine auxotroph which lacks SAT. SAT activity was detected in crude extracts of one such complemented strain. A strain of A. chroococcum carrying a chromosomal disruption of ORF4 grew normally with ammonium as the N source but more slowly than the parental strain when N2 was the sole N source. These data suggest that ORF4 encodes a nif-specific SAT required for optimizing expression of nitrogenase activity. ORF4 was assigned the name nifP. nifP may be required to boost rates of synthesis or intracellular concentrations of cysteine or methionine. Sequence identity between nifV and leuA gene products suggests that nifV may catalyze a condensation reaction analogous to that carried out by isopropylmalate synthase (LEUA) but in which acetyl coenzyme and alpha-ketoglutarate are substrates for the formation of homocitrate, the proposed product of NIFV activity.
