Evidence for functional differentiation of duplicated nifH genes in Azorhizobium caulinodans.

Azorhizobium caulinodans is a symbiotic diazotroph that contains duplicated nifH genes. This study focused on the biological sense behind the duplication. In-frame deletion mutants of nifH1 and nifH2 were constructed in order to analyze nitrogen fixation activity, both in symbiosis and in free-living conditions. Symbiotic nitrogen fixation activity was not affected by deletion of nifH1 or nifH2, while free-living nitrogen fixation activity was significantly decreased. Deletion of nifH1 had a significant effect in semi-aerobic condition, while deletion of nifH2 was significant in microaerobic condition, suggesting functional differences between nifH1 and nifH2. Transcriptional activity of nifH1 was higher than nifH2, both in microaerobic and semi-aerobic conditions.