Further analysis of nitrogen fixation (nif) genes in Azotobacter chroococcum: identification and expression in Klebsiella pneumoniae of nifS, nifV, nifM, and nifB genes and localization of nifE/N-, nifU-, nifA- and fixABC-like genes.

The results presented extend previous investigations on the genetics of nitrogen fixation in Azotobacter chroococcum and indicate that nif- and fix-like DNA is located in at least five different regions of the genome. Region I contains functional copies of nifS,V and M, as well as nifH, D and K, all of which complemented mutants of Klebsiella pneumoniae. In addition, nifE- and/or nifN-like and nifU-like DNA is located in this region. The organization of the nif cluster in region I closely resembles that of K. pneumoniae. though spread over 22 kb as compared with 14 kb. Region II contains a functional nifB gene, which complemented a K. pneumoniae nifB mutant, and seems to be adjacent to ap nifA-like gene. Region III harbours nifH*, encoding a second nitrogenase Fe-protein. Region IV contains a reiteration of nifE- on and/or nifN-like sequences, and DNA homologous to Rhizobium meliloti fixABC is present in region V. The apparent complexity of nifDNA in A. chroococcum is probably related to the two systems for N2-fixation pr present in this organism.