Light-induced carotenogenesis in Streptomyces coelicolor A3(2): identification of an extracytoplasmic function sigma factor that directs photodependent transcription of the carotenoid biosynthesis gene cluster.

Carotenoids are produced by a variety of organisms, but the mechanisms that regulate gene expression leading to carotenoid biosynthesis have been characterized for only a few organisms. In this study, we found that Streptomyces coelicolor A3(2), a gram-positive filamentous bacterium, produces carotenoids under blue light induction. The carotenoid fraction isolated from the cell extract contained multiple compounds, including isorenieratene and beta-carotene. The carotenoid biosynthesis gene cluster of S. coelicolor consists of two convergent operons, crtEIBV and crtYTU, as previously shown for Streptomyces griseus. The crtEIBV null mutant completely lost its ability to produce carotenoids. The crt gene cluster is flanked by a regulatory region that consists of two divergent operons, litRQ and litSAB. The lit (light-induced transcription) genes encode a MerR-type transcriptional regulator (LitR), a possible oxidoreductase (LitQ), an extracytoplasmic function sigma factor (sigmaLitS), a putative lipoprotein (LitA), and a putative anti-sigma factor (LitB). S1 protection assay revealed that the promoters preceding crtE (PcrtE), crtY (PcrtY), litR (PlitR), and litS (PlitS) are activated upon illumination. A litS mutant lost both the ability to produce carotenoids and the activities of PcrtE, PcrtY, and PlitS, which suggested that sigmaLitS directs light-induced transcription from these promoters. An RNA polymerase holocomplex containing purified sigmaLitS recombinant protein generated specific PcrtE and PcrtY transcripts in an in vitro runoff transcriptional assay. A litR mutant that had an insertion of the kanamycin resistance gene was defective both in the ability to produce carotenoids and in all of the light-dependent promoter activities. Overexpression of litS resulted in constitutive carotenoid production in both the wild type and the litR mutant. These results indicate that sigmaLitS acts as a light-induced sigma factor that directs transcription of the crt biosynthesis gene cluster, whose activity is controlled by an unknown LitR function. This is the first report to describe light-inducible gene expression in Streptomyces.