Hierarchical Regulation of Photosynthesis Gene Expression by the Oxygen-Responsive PrrBA and AppA-PpsR Systems of Rhodobacter sphaeroides

Abstract

In the facultatively phototrophic proteobacterium Rhodobacter sphaeroides, formation of the photosynthetic apparatus is oxygen dependent. When oxygen tension decreases, the response regulator PrrA of the global two-component PrrBA system is believed to directly activate transcription of the puf, puh, and puc operons, encoding structural proteins of the photosynthetic complexes, and to indirectly upregulate the photopigment biosynthesis genes bch and crt. Decreased oxygen also results in inactivation of the photosynthesis-specific repressor PpsR, bringing about derepression of the puc, bch, and crt operons. We uncovered a hierarchical relationship between these two regulatory systems, earlier thought to function independently. We also more accurately assessed the spectrum of gene targets of the PrrBA system. First, expression of the appA gene, encoding the PpsR antirepressor, is PrrA dependent, which establishes one level of hierarchical dominance of the PrrBA system over AppA-PpsR. Second, restoration of the appA transcript to the wild-type level is insufficient for rescuing phototrophic growth impairment of the prrA mutant, whereas inactivation of ppsR is sufficient. This suggests that in addition to controlling appA transcription, PrrA affects the activity of the AppA-PpsR system via an as yet unidentified mechanism(s). Third, PrrA directly activates several bch and crt genes, traditionally considered to be the PpsR targets. Therefore, in R. sphaeroides, the global PrrBA system regulates photosynthesis gene expression (i) by rigorous control over the photosynthesis-specific AppA-PpsR regulatory system and (ii) by extensive direct transcription activation of genes encoding structural proteins of photosynthetic complexes as well as genes encoding photopigment biosynthesis enzymes.In the facultatively phototrophic alphaproteobacterium Rhodobacter sphaeroides, photosynthesis (PS) operates under anoxic conditions. A decrease in oxygen tension triggers significant upregulation of PS gene transcription (35,38). The photosynthetic apparatus is comprised of the reaction center, encoded by the puh and puf operons, and two light-harvesting complexes, encoded by the puf and puc operons. Enzymes involved in the biosynthesis of photosynthetic pigments, i.e., bacteriochlorophyll a and carotenoids, are encoded by the bch and crt genes, respectively. Most PS-specific genes are located in the R. sphaeroides PS gene cluster, whereas the puc operons are located separately (4, 49). Three major regulatory systems control oxygen-dependent transcription of PS genes. One of these is composed of the antirepressor AppA (15, 17) and the repressor PpsR (7, 36) and is primarily responsible for the regulation of PS genes (30). Two other systems are global regulatory systems, i.e., the redox-responsive two-component system PrrBA (2, 47) and the anaerobic activator FnrL (48).

Regulation of bacterial photosynthesis genes by the small noncoding RNA PcrZ

“…More recently CryB, a member of a newly described cryptochrome family (15), was shown to affect expression of photosynthesis genes in R. sphaeroides and to interact with AppA (16,17). Remarkably, the different signaling pathways for control of photosynthesis genes are also interconnected, e.g., the appA gene is controlled by PrrA (18,19) and a PpsR binding site is located in the ppaA promoter region (20). Thus, a complex network comprising several regulatory proteins controls the formation of the photosynthetic apparatus.…”

The PpaA/AerR Regulators of Photosynthesis Gene Expression from Anoxygenic Phototrophic Proteobacteria Contain Heme-Binding SCHIC Domains

“…Vitamin B 12 is not synthesized by E. coli but can be taken up from the medium, as evidenced by the pinkish color of the anaerobically grown E. coli DH5␣ cells and presence of cobalamin in crude cell extracts (data not shown). We found that MBP-SCHIC Rs purified from these cells and cells not exposed to cyanocobalamin had identical spectra indicative of bound heme, not cyanocobalamin (Fig.…”

“…To compare the affinities of MBP-SCHIC Rs for heme and vitamin B 12 , we expressed MBP-SCHIC Rs in E. coli grown anaerobically in media supplemented with high (50 M) concentration of vitamin B 12 (cyanocobalamin; Sigma). Vitamin B 12 is not synthesized by E. coli but can be taken up from the medium, as evidenced by the pinkish color of the anaerobically grown E. coli DH5␣ cells and presence of cobalamin in crude cell extracts (data not shown).…”

Oxygen-Dependent Regulation of Bacterial Lipid Production

“…The pathways that control transcription of many genes required for the synthesis and function of the photosynthetic apparatus have been extensively studied at the genetic and genomic levels (12)(13)(14)(15). The photosynthesis response regulator (Prr) proteins comprise a two-component signal transduction system activated by low O 2 tension, in which PrrB is a membrane-spanning sensor histidine kinase and PrrA is a DNA-binding response reg-ulator (16,17).…”