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

Blue light triggers carotenogenesis in the nonphototrophic bacterium Myxococcus xanthus by inducing inactivation of an anti-sigma factor, CarR, and the consequent liberation of the cognate extracytoplasmic function (ECF) sigma 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 (O-1(2)) by energy transfer to oxygen. However, O-1(2) involvement in M. xanthus light-induced carotenogenesis remains to be established. Here, we present genetic evidence of the involvement of PPIX as well as O-1(2) 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 O-1(2) and is inhibited by exogenously supplied hemin and that CarF is essential for the action of O-1(2). Thus, our findings indicate that blue light interaction with PPIX generates O-1(2), which must be transmitted via CarF to trigger the transcriptional response underlying light-induced carotenogenesis in M. xanthus.