Molecular genetic analysis suggesting interactions between AppA and PpsR in regulation of photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1.

The AppA protein plays an essential regulatory role in development of the photosynthetic apparatus in the anoxygenic phototrophic bacterium Rhodobacter sphaeroides 2.4.1 (M. Gomelsky and S. Kaplan, J. Bacteriol. 177:4609-4618, 1995). To gain additional insight into both the role and site of action of AppA in the regulatory network governing photosynthesis gene expression, we investigated the relationships between AppA and other known regulators of photosynthesis gene expression. We determined that AppA is dispensable for development of the photosynthetic apparatus in a ppsR null background, where PpsR is an aerobic repressor of genes involved in photopigment biosynthesis and puc operon expression. Moreover, all suppressors of an appA null mutation thus far isolated, showing improved photosynthetic growth, were found to contain mutations in the ppsR gene. Because ppsR gene expression in R. sphaeroides 2.4.1 appears to be largely independent of growth conditions, we suggest that regulation of repressor activity occurs predominately at the protein level. We have also found that PpsR functions as a repressor not only under aerobic but under anaerobic photosynthetic conditions and thereby is involved in regulating the abundance of the light harvesting complex II, depending on light intensity. It seems likely therefore, that PpsR responds to an integral signal (e.g., changes in redox potential) produced either by changes in oxygen tension or light intensity. The profile of the isolated suppressor mutations in PpsR is in accord with this proposition. We propose that AppA may be involved in a redox-dependent modulation of PpsR repressor activity.