AerR, a Second Aerobic Repressor of Photosynthesis Gene Expression in Rhodobacter capsulatus

Abstract

Open reading frame orf192, which is located immediately upstream of the aerobic repressor gene crtJ, was genetically and biochemically demonstrated to code for a second aerobic repressor (AerR) of photosynthesis gene expression in Rhodobacter capsulatus. Promoter-mapping studies indicate that crtJ has its own promoter but that a significant proportion of crtJ expression is promoted by read-through transcription of orf192 (aerR) transcripts through crtJ. Disruption of aerR resulted in increased photopigment biosynthesis during aerobic growth to a level similar to that of disruption of crtJ. Like that reported for CrtJ, ␤-galactosidase assays of reporter gene expression indicated that disruption of aerR resulted in a two-to threefold increase in aerobic expression of the crtI and pucB operons. However, unlike CrtJ, AerR aerobically represses puf operon expression and does not aerobically repress bchC expression. Gel mobility shift analysis with purified AerR indicates that AerR does not bind to a bchC promoter probe but does bind to the crtI, puc, and puf promoter probes. These results indicate that AerR is a DNA-binding protein that targets genes partially overlapping a subset of genes that are also controlled by CrtJ. We also provide evidence for cooperative binding of AerR and CrtJ to the puc promoter region.Oxygen tension is an important factor regulating synthesis and assembly of the photosynthetic apparatus in photosynthetic bacteria (7). Oxygen suppression of photosystem synthesis is mainly attributed to the activation or inactivation of many transcription factors that regulate photosynthesis gene expression. One well-characterized, oxygen-regulated transcription factor is the aerobic repressor CrtJ, which in the presence of oxygen represses bacteriochlorophyll (bch), carotenoid (crt), and respiratory (29) and light harvesting-II (puc) (22) gene expression. It has recently been demonstrated that exposure of CrtJ to oxygen results in the formation of an intramolecular disulfide bond and that formation of this bond is needed for binding of CrtJ to its target promoters (S. Masuda, C. Dong, D. Swem, and C. E. Bauer, unpublished data). Another wellcharacterized redox regulator is the sensor kinase RegB, which, together with its cognate response regulator RegA, is responsible for the global control of many aerobically and anaerobically regulated cellular processes (3). This includes photosynthesis (17, 26), nitrogen fixation (8), hydrogen utilization (8), carbon fixation (32), respiration (29, 30), and cytochrome biosynthesis (29,30).In this study, we have identified a new transcription factor coded by an open reading frame (orf192) located just upstream of crtJ in the previously sequenced Rhodobacter capsulatus photosynthesis gene cluster (1). Disruption of orf192 indicates that it codes for an aerobic repressor (AerR) of carotenoids and bacteriochlorophyll and for the reaction center and light-harvesting apoproteins. The results of gel mobility shift assays indicate that AerR binds to the pufQ, crtA-crtI, and p...

Coordination of Ubiquinol Oxidase and Cytochrome cbb 3 Oxidase Expression by Multiple Regulators in Rhodobacter capsulatus

“…Analysis of ubiquinol oxidase expression indicates that aerR and crtJ mutants exhibit a twofold increase in expression aerobically and no effect anaerobically. This is very similar to the effect on bch, crt, and puc expression that is also observed upon disruption of these regulators (5,8).…”

“…Specifically, aerobic repression of ubiquinol oxidase appears to be promoted by the photopigment repressors CrtJ and AerR. These two repressors are known to be responsible for aerobic repression of bacteriochlorophyll, carotenoid, light harvesting, and reaction center gene expression (5,8,17). Our observation that CrtJ and AerR also aerobically repress expression of ubiquinol oxidase (Fig.…”

Identification and in vivo characterization of PpaA, a regulator of photosystem formation in Rhodobacter sphaeroides

“…In contrast, the PpaA homologue from R. capsulatus, AerR (ORF192), was recently reported to function as an aerobic repressor of a subset of PS genes (Dong et al, 2002). In a search for clues regarding PpaA function, we turned to the protein sequence analysis.…”

Repression of photosynthesis gene expression by formation of a disulfide bond in CrtJ

“…For in vivo analysis of disulfide bonds, a strain of Rhodobacter capsulatus was used (ES7 CrtJ-flag) that expressed a chromosomally encoded S-Tag variant of CrtJ (11). Spectral and gel mobility shift data indicated that addition of an S tag at the amino terminus does not affect repressing properties of Crt J (data not shown).…”