A Redox-Responsive Pathway for Aerobic Regulation of Photosynthesis Gene Expression in Rhodobacter sphaeroides 2.4.1

The facultative photoheterotroph Rhodobacter sphaeroides 2.4.1 displays a remarkable ability to grow under a variety of environmental conditions, being capable of aerobic, anaerobic, and photosynthetic growth and of fixing atmospheric nitrogen and carbon dioxide (15). Significant progress has been made in understanding molecular aspects of the regulatory pathways which make this adaptability possible, and many of the key components involved in the regulation of photosystem formation have been identified. However, much less is known about interactions between the regulatory pathways themselves. At least four such regulatory pathways in R. sphaeroides are known to be responsive to oxygen, with the FnrL protein and the Prr two-component system acting in a positive manner to control the expression of many pigment and light-harvesting-complex genes (4–6, 38), while the repressor PpsR negatively regulates photosynthesis gene (PS gene) expression (9). The TspO regulatory network appears to generate a signal through which a number of pigment biosynthesis genes and the puc operon are also negatively regulated (33, 34). Recently, we reported that the membrane-bound RdxBH proteins and the cbb3 cytochrome oxidase are involved in providing a signal through which aerobic photosynthesis gene expression is repressed. These same redox-active proteins also play a role in governing the relative accumulation of the carotenoids (Crt) spheroidene (SE) and spheroidenone (SO) under anaerobic conditions (24). Because the cbb3-generated signal normally acts to inhibit PS gene expression under aerobic conditions, it was reasoned that its target is a positive-acting regulatory effector, namely the Prr system. The genetic region encoding the prr locus comprises three genes. The prrA gene is predicted to encode a cytoplasmic regulator, while prrB appears to encode a membrane-bound histidine kinase. Together, PrrA and PrrB represent a two-component signal transduction system. PrrA mutants are photosynthetically incompetent, while PrrB mutants do grow photosynthetically, but only under high light intensity. It was genetically shown that PrrA can be activated by phosphoryl donors other than PrrB (6, 10). The prrC gene is predicted to encode a membrane-associated protein somewhat analogous to a yeast cytochrome oxidase assembly factor (5); however, a specific role for PrrC has not yet been identified, although it does appear to be involved in this signal transduction pathway, as observed when deletions affecting different lengths of prrC are used (5). It has previously been proposed that PrrB may directly sense oxygen or the redox state of some other membrane component(s) (5); however, the precise mechanism(s) through which oxygen influences PrrB activity remains to be determined. In order to examine the relationship between Prr- and cbb3-RdxBH-dependent PS gene expression and carotenoid biosynthesis, we have constructed a series of mutants lacking components of the Prr system and either the cbb3-type cytochrome oxidase or the RdxBH proteins. The production of both light-harvesting complexes and the transcriptional activity of the puc and puf operons in cbb3-PRR double mutants have been examined under aerobic and anaerobic conditions. By using a CCOP1PRRB1 double mutant, the accumulation of the carotenoids SE and SO under high-light photosynthetic conditions was also measured. The data presented in this paper indicate that the RdxBH proteins and cbb3 cytochrome oxidase can be categorized as new members of a complex pathway, represented by the Prr two-component system, which plays a central role in the regulation of PS gene expression and photopigment biosynthesis. Together, these constitute a sensor-signal transduction pathway, positively regulating PS gene expression.