Figure 2. Regulation model for oxygen responsive sRNAs in R. sphaeroides. Reactive oxygen species are either generated by energy transfer during photosynthesis or due to electron transfer mediated by flavoenzymes during aerobic respiration. Several alternative sigma factors (RpoE, RpoHI, RpoHII) are activated, which subsequently induce sRNAs. During aerobic respiration, phosphorylation of the response regulator PrrA by the sensor kinase PrrB is inhibited due to electron flow through the ccb3 oxidase. Under low oxygen tension, PrrA is phosphorylated and triggers PcrZ transcription, which in turn, represses photosynthesis gene expression. Interaction of some sRNAs with Hfq is indicated. (LH, light harvesting complex; RC, reaction center; UQ, ubiquinone pool; Cyt, cytochrome; P, phosphate, DH, dehydrogenase; FADH2, reduced flavin adenine dinucleotide; FFL, feed-forward loop; 1O2, singlet oxygen; O2‒, superoxide radical; H2O2, hydrogen peroxide).