Regulation of expression of genes for light-harvesting antenna proteins LH-I and LH-II; reaction center polypeptides RC-L, RC-M, and RC-H; and enzymes of bacteriochlorophyll and carotenoid biosynthesis in Rhodobacter capsulatus by light and oxygen.

RNA levels were measured by blot hybridization to study the coordinate and differential expression of Rhodobacter capsulatus genes for light-harvesting I antenna proteins LH-I and LH-II; reaction center (RC) polypeptides L, M, and H; and bacteriochlorophyll and carotenoid biosynthesis in response to light and O2. The genes for LH-II alpha and beta subunits only have one transcript, 0.5 kilobase (kb) long, whereas the genes for LH-I have two transcripts (0.5 and 2.6 kb). The small transcript (0.5 kb) is the mRNA only for LH-I beta and alpha polypeptides, whereas the large transcript (2.6 kb) codes for RC-L, RC-M, and the beta and alpha polypeptides of LH-I, as well as the product of an unknown open reading frame designated ORF C2397. These five genes thus comprise a single operon (designated the puf operon). The mRNA specifying the LH-II polypeptides is more abundant, more sensitive to changes in O2 concentration, and shows a variation over a wider range than that of the mRNA for LH-I, indicating that the genes for LH-II and LH-I/RC are regulated independently. The gene for RC-H (puhA) has at least two transcripts (1.2 and 1.4 kb) that initiate within ORF F1696 and respond differentially to light intensity. The expression of the genes coding for RC-L, RC-M, and RC-H is coordinately regulated by light intensity and O2 concentration. An increase in light intensity causes a decrease in the expression of the genes for LH-I, LH-II, and RC proteins. The genes coding for the enzymes in the bacteriochlorophyll biosynthetic pathways are regulated by light intensity and O2 in a manner similar to that of the genes for LH and RC proteins. The crt genes coding for the enzymes in carotenoid biosynthetic pathways, however, are regulated in an opposite fashion: high light intensity results in increased expression of crt genes. These results are interpreted based on the protective function of carotenoids under high light intensity in the presence of O2.