AEROBIC AND ANAEROBIC REGULATION IN RHODOBACTER-SPHAEROIDES-2.4.1 - THE ROLE OF THE FNRL GENE

In Rhodobacter sphaeroides 2.4.1, the cellular requirements for 5-amin olevulinic acid (ALA) are in part regulated by the level of ALA syntha se activity, which is encoded by the hemA and hemT genes. Under standa rd growth conditions, only the hemA gene is transcribed, and the level of ALA synthase activity varies in response to oxygen tension. The pr esence of an FNR consensus sequence upstream of hemA Suggested that ox ygen regulation of hemA expression could be mediated, in part, through a homolog of the fur gene, Two independent studies, one detailed here , identified a region of the R, sphaeroides 2.4.1 genome containing ex tensive homology to the fix region of the symbiotic nitrogen-fixing ba cteria Rhizobium meliloti and Bradyrhizobium japonicum. Within this re gion that maps to 443 kbp on chromosome I, we have identified an fnr h omolog (fnrL), as well as a gene that codes for an anaerobic coproporp hyrinogen III oxidase, the second such gene identified in this organis m. We alsb present an analysis of the role of fnrL in the physiology o f R. sphaeroides 2.4.1 through the construction and characterization o f fnrL-null strains, Our results further show that fnrL is essential f or both photosynthetic and anaerobic-dark growth with dimethyl sulfoxi de, Analysis of hemA expression, with hemA::lacZ transcriptional fusio ns, suggests that FnrL is an activator of hemA under anaerobic conditi ons, On the other hand, the open reading frame immediately upstream of hemA appears to be an activator of hemA transcription regardless of e ither the presence or the absence of oxygen or FnrL, Given the lack of hemT expression under these conditions, we consider FnrL regulation o f hemA expression to be a major factor in bringing about changes in th e level of ALA synthase activity in response to changes in oxygen tens ion.