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 photos ynthesis gene expression, we investigated the relationships between Ap pA and other known regulators of photosynthesis gene expression, We de termined that AppA is dispensable for development of the photosyntheti c apparatus in a ppsR null background, where PpsR is an aerobic repres sor of genes involved in photopigment biosynthesis and pac operon expr ession, Moreover, all suppressors of an appA null mutation thus far is olated, showing improved photosynthetic growth, were found to contain mutations in the ppsR gene, Because ppsR gene expression in R. sphaero ides 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 repress or not only under aerobic but under anaerobic photosynthetic condition s and thereby is involved in regulating the abundance of the light har vesting complex II, depending on light intensity, It seems likely ther efore, that PpsR responds to an integral signal (e.g., changes in redo x potential) produced either by changes in oxygen tension or light int ensity, The profile of the isolated suppressor mutations in PpsR is in accord with this proposition, We propose that AppA mag be involved in a redox-dependent modulation of PpsR repressor activity.