NEGATIVE AUTOREGULATION OF THE RHIZOBIUM-MELILOTI FIXK GENE IS INDIRECT AND REQUIRES A NEWLY IDENTIFIED REGULATOR, FIXT

fixK genes are crpl fnr homologues that have been discovered in divers e Rhizobium spp., in which they are usually essential for symbiotic ni trogen fixation, One recurrent function of fixK genes in rhizobia is t o activate the transcription of operons required for respiration in th e microoxic environment of the nodule,ln a similar manner to its Esche richia coli crp and fnr homologues, R. meliloti fixK regulates its own expression negatively, However, we demonstrate here that fixK negativ e autoregulation is not direct and, instead, involves a newly identifi ed gene, fixT, the expression of which depends on fixK. Inactivation o f fixT resulted in derepression of fixK expression under free-living m icrooxic conditions, Furthermore, constitutively expressed fixT strong ly repressed fixK-lacZ expression in the absence of a functional fixK gene. Several lines of evidence indicate that fixT is active via its p rotein product FixT. FixT does not resemble any protein present in dat abases so far, Modules induced by a fixT mutant were Fix(+), thus demo nstrating that fixT is not essential for symbiotic nitrogen fixation.