Overlapping Alternative Sigma Factor Regulons in the Response to Singlet Oxygen in Rhodobacter sphaeroides

Organisms performing photosynthesis in the presence of oxygen have to cope with the formation of highly reactive singlet oxygen ((1)O(2)) and need to mount an adaptive response to photooxidative stress. Here we show that the alternative sigma factors RpoH(I) and RpoH(II) are both involved in the (1)O(2) response and in the heat stress response in Rhodobacter sphaeroides. We propose RpoH(II) to be the major player in the (1)O(2) response, whereas RpoH(I) is more important for the heat stress response. Mapping of the 5' ends of RpoH(II)- and also RpoH(I)/RpoH(II)-dependent transcripts revealed clear differences in the -10 regions of the putative promoter sequences. By using bioinformatic tools, we extended the RpoH(II) regulon, which includes genes induced by 1O2 exposure. These genes encode proteins which are, e. g., involved in methionine sulfoxide reduction and in maintaining the quinone pool. Furthermore, we identified small RNAs which depend on RpoH(I) and RpoH(II) and are likely to contribute to the defense against photooxidative stress and heat stress.