Symbiotic Terminal Oxidase CBB3 Production is Negatively Modulated by a Purine Related Metabolite in Sinorhizobium Meliloti

Abstract

The genes which code for a bacteroid terminal oxidase, denoted as the cytochrome terminal oxidase cbb ₃ have been identified as the fixNOQP operon. In Sinorhizobium meliloti, the expression of the fixNOQP genes is mainly regulated in response to O₂ availability, through the oxygen sensing cascade fixL, fixJ, and fixK gene products. fixL codes for a membrane bound haem-containing protein which in response to low O₂ phosphorylates itself and FixJ, activating the capacity of FixJ to promote the transcription of fixK. In turn FixK activates the transcription of the fixNOQP operon and negatively regulates its own promoter (Batut et al., 1989). fixK codes for a transcriptional activator homologous to Fnr and Crp (Batut et al., 1989). The nature of the possible effector which interacts with FixK is unknown. In R. leguminosarum biovar viciae, an homologous but different transcriptional activator, FnrN, activates the two fixNOQP copies (Collona-Romano et al., 1990; Patschowski et al., 1996; Gutierrez et al., 1997). In contrast with FixK, FnrN conserves the N-terminal domain involved in O₂ sensing present in Fnr from E. colt suggesting that in R. leguminosarum FnrN transcriptional activity is negatively modulated by O₂ (Collona-Romano et al., 1990). Analysis of the transcription of the fixNOQP operon in different R. etli mutants of the purine-thiamin biosynthetic pathway, suggested that 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) is a negative effector of the expression of these genes (Soberón et al., 1997; Miranda-Rios et al., 1997).