In vitro and in vivo analysis of the role of PrrA in Rhodobacter sphaeroides 2.4.1 hemA gene expression.

The hemA gene codes for one of two synthases in Rhodobacter sphaeroides 2.4.1 which catalyze the formation of 5-aminolevulinic acid. We have examined the role of PrrA, a DNA binding protein that is associated with the metabolic switch between aerobic growth and anoxygenic photosynthetic growth, in hemA expression and found that hemA transcription is directly activated by PrrA. Using electrophoretic mobility shift assays and DNase I protection assays, we have mapped two binding sites for PrrA within the hemA upstream sequences, each of which contains an identical 9-bp motif. Using lacZ transcription reporter plasmids in wild-type strain 2.4.1 and PrrA- mutant strain PRRA2, we showed that PrrA was required for maximal expression. We also found that the relative impacts of altering DNA sequences within the two binding sites are different depending on whether cells are growing aerobically or anaerobically. This reveals a greater level of complexity associated with PrrA-mediated regulation of transcription than has been heretofore described. Our findings are of particular importance with respect to those genes regulated by PrrA having more than one upstream binding site. In the case of the hemA gene, we discuss possibilities as to how these new insights can be accommodated within the context of what has already been established for hemA transcription regulation in R. sphaeroides.