RpoH II Activates Oxidative-Stress Defense Systems and Is Controlled by RpoE in the Singlet Oxygen-Dependent Response in Rhodobacter sphaeroides

Abstract

Photosynthetic organisms need defense systems against photooxidative stress caused by the generation of highly reactive singlet oxygen ( ¹ O ₂ ). Here we show that the alternative sigma factor RpoH _II is required for the expression of important defense factors and that deletion of rpoH _II leads to increased sensitivity against exposure to ¹ O ₂ and methylglyoxal in Rhodobacter sphaeroides . The gene encoding RpoH _II is controlled by RpoE, and thereby a sigma factor cascade is constituted. We provide the first in vivo study that identifies genes controlled by an RpoH _II -type sigma factor, which is widely distributed in the Alphaproteobacteria . RpoH _II -dependent genes encode oxidative-stress defense systems, including proteins for the degradation of methylglyoxal, detoxification of peroxides, ¹ O ₂ scavenging, and redox and iron homeostasis. Our experiments indicate that glutathione (GSH)-dependent mechanisms are involved in the defense against photooxidative stress in photosynthetic bacteria. Therefore, we conclude that systems pivotal for the organism's defense against photooxidative stress are strongly dependent on GSH and are specifically recognized by RpoH _II in R. sphaeroides .