Evidence that Altered <i>Cis</i> Element Spacing Affects PpsR Mediated Redox Control of Photosynthesis Gene Expression in <i>Rubrivivax gelatinosus</i>

<div><p>PpsR is a major regulator of photosynthesis gene expression among all characterized purple photosynthetic bacteria. This transcription regulator has been extensively characterized in <i>Rhodobacter</i> (<i>Rba</i>.) <i>capsulatus</i> and <i>Rba</i>. <i>sphaeroides</i> which are members of the α-proteobacteria lineage. In this study, we have investigated the biochemical properties and mutational effects of a <i>ppsR</i> deletion strain in the β-proteobacterium <i>Rubrivivax</i> (<i>Rvi</i>.) <i>gelatinosus</i> in order to reveal phylogenetically conserved mechanisms and species-specific characteristics. A deletion of the <i>ppsR</i> gene resulted in de-repression of photosystem synthesis showing that PpsR functions as a repressor of photosynthesis genes in this species. We also constructed a <i>Rvi</i>. <i>gelatinosus</i> PpsR mutant in which a conserved cysteine at position 436 was changed to an alanine to examine whether or not this residue is important for sensing redox, as reported in <i>Rhodobacter</i> species. Surprisingly, the Cys436 Ala mutant retained the ability to repress photosynthesis gene expression under aerobic conditions, suggesting that PpsR from <i>Rvi</i>. <i>gelatinosus</i> has different redox-responding characteristics. Furthermore, biochemical analyses demonstrated that <i>Rvi</i>. <i>gelatinosus</i> PpsR only shows redox-dependent binding to promoters with 9-bp spacing, but not 8-bp spacing, between two PpsR-recognition sequences. These results indicate that redox-dependent binding of PpsR requires appropriate <i>cis</i> configuration of PpsR target sequences in <i>Rvi</i>. <i>gelatinosus</i>. These results also indicate that PpsR homologs from different species regulate photosynthesis genes with altered biochemical properties.</p>