CarF Mediates Signaling by Singlet Oxygen, Generated via Photoexcited Protoporphyrin IX, in Myxococcus xanthus Light-Induced Carotenogenesis

摘要

Blue light triggers carotenogenesis in the nonphototrophic bacterium Myxococcus xanthus by inducing inactivation of an anti-σ factor, CarR, and the consequent liberation of the cognate extracytoplasmic function (ECF) σ factor, CarQ. CarF, the protein implicated earliest in the response to light, does not resemble any known photoreceptor. It interacts physically with CarR and is required for its light-driven inactivation, but the mechanism is unknown. Blue-light sensing in M. xanthus has been attributed to the heme precursor protoporphyrin IX (PPIX), which can generate the highly reactive singlet oxygen species (1O₂) by energy transfer to oxygen. However, 1O₂ involvement in M. xanthus light-induced carotenogenesis remains to be established. Here, we present genetic evidence of the involvement of PPIX as well as 1O₂ in light-induced carotenogenesis in M. xanthus and of how these are linked to CarF in the signal transduction pathway. Response to light was examined in carF-bearing and carF-deficient M. xanthus strains lacking endogenous PPIX due to deletion of hemB or accumulating PPIX due to deletion of hemH (hemB and hemH are early- and late-acting heme biosynthesis genes, respectively). This demonstrated that light induction of the CarQ-dependent promoter, P_QRS, correlated directly with cellular PPIX levels. Furthermore, we show that P_QRS activation is triggered by 1O₂ and is inhibited by exogenously supplied hemin and that CarF is essential for the action of 1O₂. Thus, our findings indicate that blue light interaction with PPIX generates 1O₂, which must be transmitted via CarF to trigger the transcriptional response underlying light-induced carotenogenesis in M. xanthus.