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-aminolevulinic acid (ALA) are in part regulated by the level of ALA synthase activity, which is encoded by the hemA and hemT genes. Under standard growth conditions, only the hemA gene is transcribed, and the level of ALA synthase activity varies in response to oxygen tension. The presence of an FNR consensus sequence upstream of hemA suggested that oxygen regulation of hemA expression could be mediated, in part, through a homolog of the fnr gene. Two independent studies, one detailed here, identified a region of the R. sphaeroides 2.4.1 genome containing extensive homology to the fix region of the symbiotic nitrogen-fixing bacteria Rhizobium meliloti and Bradyrhizobium japonicum. Within this region that maps to 443 kbp on chromsome I, we have identified an fnr homolog (fnrL), as well as a gene that codes for an anaerobic coproporphyrinogen III oxidase, the second such gene identified in this organism. We also present an analysis of the role of fnrL in the physiology of R. sphaeroides 2.4.1 through the construction and characterization of fnrL-null strains. Our results further show that fnrL is essential for both photosynthetic and anaerobic-dark growth with dimethyl sulfoxide. Analysis of hemA expression, with hemA::lacZ transcriptional fusions, suggests that FnrL is an activator of hemA under anaerobic conditions. On the other hand, the open reading frame immediately upstream of hemA appears to be an activator of hemA transcription regardless of either the presence or the absence of oxygen or FnrL. Given the lack of hemT expression under these conditions, we consider FnrL regulation of hemA expression to be a major factor in bringing about changes in the level of ALA synthase activity in response to changes in oxygen tension.