Negative autoregulation of the Rhizobium meliloti fixK gene is indirect and requires a newly identified regulator, FixT.

fixK genes are crp/fnr homologues that have been discovered in diverse Rhizobium spp., in which they are usually essential for symbiotic nitrogen fixation. One recurrent function of fixK genes in rhizobia is to activate the transcription of operons required for respiration in the microoxic environment of the nodule. In a similar manner to its Escherichia coli crp and fnr homologues, R. meliloti fixK regulates its own expression negatively. However, we demonstrate here that fixK negative autoregulation is not direct and, instead, involves a newly identified gene, fixT, the expression of which depends on fixK. Inactivation of fixT resulted in derepression of fixK expression under free-living microoxic conditions. Furthermore, constitutively expressed fixT strongly repressed fixK–lacZ expression in the absence of a functional fixK gene. Several lines of evidence indicate that fixT is active via its protein product FixT. FixT does not resemble any protein present in databases so far. Nodules induced by a fixT mutant were Fix+, thus demonstrating that fixT is not essential for symbiotic nitrogen fixation.