UV-B radiation induced exchange of the D1 reaction centre subunits produced from the psbA2 and psbA3 genes in the Cyanobacterium synechocystis sp. PCC 6803.

UV-B irradiation of Synechocystis 6803 cells inhibits photosystem II activity, which can be restored via de novo synthesis of the D1 (and D2) reaction center subunits. Recently we have shown that of the two psbA genes that encode identical D1 proteins in Synechocystis 6803, UV-B preferentially enhances the transcription of psbA3 compared to that of psbA2 [Máté, Z., Sass, L., Szekeres, M., Vass, I. and Nagy, F. (1998) J. Biol. Chem. 273, 17439-17444]. Here we studied the effect of UV-B on the synthesis of the D1 protein from the psbA2 and psbA3 genes in the P7 mutant of Synechocystis 6803. In this mutant, psbA2 carries the Ala251-->Val point mutation, which confers resistance to the photosystem II electron transport inhibitor metribuzin, but psbA3 is the same as in the wild-type. By applying variable chlorophyll fluorescence measurements to distinguish between metribuzin-sensitive and metribuzin-resistant photosystem II centers we quantified the amount of the D1 protein produced from each of the psbA3 and psbA2 genes. When the cells were exposed to UV-B light, the fraction of D1 protein produced from the psbA3 gene was increased from 15-20 to 32-40% of the total D1. This effect was reversible by transferring the cells to visible light. The rate of D1 production from psbA3 increased with increasing UV-B intensities, and was a transient phenomenon at low UV-B levels (0.1 microE x m-2 x s-1). It is concluded that the enhancement of psbA3 gene transcription by UV-B light leads to enhanced D1 protein synthesis from this gene. Our findings demonstrate that the main role of psbA3 transcription activated by UV-B is to increase the size of the psbA mRNA pool available for translation when a rapid repair of the D1 protein is needed under UV-B stress.