SUMMARY: The results presented extend previous investigations on the genetics of nitrogen fixation in and indicate that and -like DNA is located in at least five different regions of the genome. Region I contains functional copies of and , as well as and , all of which complemented mutants of In addition, -and/or -like and -like DNA is located in this region. The organization of the cluster in region I closely resembles that of , though spread over 22 kb as compared with 14 kb. Region II contains a functional gene, which complemented a mutant, and seems to be adjacent to a -like gene. Region III harbours , encoding a second nitrogenase Fe-protein. Region IV contains a reiteration of - and/or -like sequences, and DNA homologous to is present in region V. The apparent complexity of DNA in is probably related to the two systems for N-fixation present in this organism.


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