Control of hemA expression in Rhodobacter sphaeroides 2.4.1: regulation through alterations in the cellular redox state.

Rhodobacter sphaeroides 2.4.1 has the ability to synthesize a variety of tetrapyrroles, reflecting the metabolic versatility of this organism and making it capable of aerobic, anaerobic, photosynthetic, and diazotrophic growth. The hemA and hemT genes encode isozymes that catalyze the formation of 5-aminolevulinic acid, the first step in the biosynthesis of all tetrapyrroles present in R. sphaeroides 2.4.1. As part of our studies of the regulation and expression of these genes, we developed a genetic selection that uses transposon mutagenesis to identify loci affecting the aerobic expression of the hemA gene. In developing this selection, we found that sequences constituting an open reading frame immediately upstream of hemA positively affect hemA transcription. Using a transposon-based selection for increased hemA expression in the absence of the upstream open reading frame, we isolated three independent mutants. We have determined that the transposon insertions in these strains map to three different loci located on chromosome 1. One of the transposition sites mapped in the vicinity of the recently identified R. sphaeroides 2.4.1 homolog of the anaerobic regulatory gene fnr. By marker rescue and DNA sequence analysis, we found that the transposition site was located between the first two genes of the cco operon in R. sphaeroides 2.4.1, which encodes a cytochrome c terminal oxidase. Examination of the phenotype of the mutant strain revealed that, in addition to increased aerobic expression of hemA, the transposition event also conferred an oxygen-insensitive development of the photosynthetic membranes. We propose that the insertion of the transposon in cells grown in the presence of high oxygen levels has led to the generation of a cellular redox state resembling either reduced oxygen or anaerobiosis, thereby resulting in increased expression of hemA, as well as the accumulation of spectral complex formation. Several models are presented to explain these findings.