| Summary: | Photosynthetic bacteria have to deal with the risk of photooxidative stress that occurs in presence of light and oxygen due to the photosensitizing activity of (bacterio-) chlorophylls. Facultative phototrophs of the genus <i>Rhodobacter</i> adapt the formation of photosynthetic complexes to oxygen and light conditions, but cannot completely avoid this stress if environmental conditions suddenly change. <i>R. capsulatus</i> has a stronger pigmentation and faster switches to phototrophic growth than <i>R. sphaeroides</i>. However, its photooxidative stress response has not been investigated. Here, we compare both species by transcriptomics and proteomics, revealing that proteins involved in oxidation−reduction processes, DNA, and protein damage repair play pivotal roles. These functions are likely universal to many phototrophs. Furthermore, the alternative sigma factors RpoE and RpoH<sub>II</sub> are induced in both species, even though the genetic localization of the <i>rpoE</i> gene, the RpoE protein itself, and probably its regulon, are different. Despite sharing the same habitats, our findings also suggest individual strategies. The <i>crtIB-tspO</i> operon, encoding proteins for biosynthesis of carotenoid precursors and a regulator of photosynthesis, and <i>cbiX</i>, encoding a putative ferrochelatase, are induced in <i>R. capsulatus</i>. This specific response might support adaptation by maintaining high carotenoid-to-bacteriochlorophyll ratios and preventing the accumulation of porphyrin-derived photosensitizers.
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