Carotenoids are produced by all photosynthetic organisms and a large number\ud of bacteria and fungi. They are responsible for a lot of pigmentation in nature, as well\ud as often playing an essential role in the provision of light protection to cells and as\ud precursors of vitamin A in higher organisms. Myxobacton is the primary carotenoid\ud ester generated in the photoresistant bacterium Myxococcus xanthus. It is created\ud through a complex light-regulated gene expression cascade and acts to protect the\ud bacteria from blue light and the resultant generation of damaging singlet oxygen\ud species in the presence of porphyrins. The final stage in its production is a\ud ketonisation, and the enzyme responsible for this stage was unknown in M. xanthus.\ud We propose a possible location for the gene encoding such a ketolase, crtW. The\ud gene is found located within a four-gene operon separate from the other known\ud carotenoid biosynthetic genes, and appears to have two alternative promoter regions.\ud The additional genes in the operon were found to encode a putative MutT/Nudix\ud family hydrolase and a periplasmic, molybdopterin and haeme-dependent\ud oxidoreductase, YedYZ. It is also shown that crtW transcription is independent of\ud cell exposure to blue light and that the product is an inner membrane, integral\ud membrane protein probable ketolase. The evolutionary origins of crtW are considered\ud in conjunction with a number of other carotenoid biosynthetic enzymes, suggesting\ud that the gene was one of the last to be acquired by M. xanthus
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