Rhizobia are aerobic soil bacteria capable of infecting legume root nodules. Inside root nodules, rhizobia differentiate into bacteroids and express the O2-sensitive nitrogenase enzyme, reducing atmospheric dinitrogen into ammonia which is then secreted to the host plant for assimilation. Nitrogen fixation is activated in bacteroids by low O2 tension within nodules, and across the various genera of rhizobia, O2 status is known to be sensed by a range of heme-containing sensor-regulator systems. Rhizobia in the genus Mesorhizobium express the novel LacI/GalR-type regulator FixV. Even though FixV lacks an O2-binding heme group, it has been suggested that it is a key regulator of nitrogen fixation in this group of rhizobia. However, nitrogenase activity of a ΔfixV mutant of the chickpea-nodulating strain CC1192 was reduced, but not abolished, indicating that some other regulatory mechanism must control nitrogen fixation in this strain, with FnrN which has recently been shown to sense low O2 tension in Rhizobium spp., a likely contender. To investigate the role of FnrN within CC1192, a site-directed marker exchange mutant was constructed and confirmed by Illumina whole genome sequencing. CC1192 also harbours a second potential O2 sensor-regulator system (FixLJ-FixK) on its 648-kb repABC-type plasmid pMc1192, and this plasmid was cured from both ΔfnrN and ΔfixV using a plasmid incompatibility approach. In glasshouse trials with chickpea, nitrogenase activity of ΔfixV and ΔfixV plasmid cured strains were reduced to 25% of the wild-type rate, indicating the plasmid was not influencing rates of nitrogen fixation. In contrast, nitrogenase activity was reduced to 11% for ΔfnrN mutant and less than 1% for the ΔfnrN plasmid cured strain, relative to the wild-type rate, suggesting a synergistic effect may exist between FnrN and FixLJ-FixK system. Further work will investigate this possibility by creating double fnrN and fixLJ-fixK mutants and through RNAseq interrogation of the symbiotic transcriptome of CC1192.