UV-B-INDUCED DIFFERENTIAL TRANSCRIPTION OF PSBA GENES ENCODING THE D1PROTEIN OF PHOTOSYSTEM-II IN THE CYANOBACTERIUM SYNECHOCYSTIS-6803

UV-B irradiation of intact Synechocystis sp, PCC 6803 cells results in the loss of photosystem II activity, which can be repaired via de nov o synthesis of the D1 land D2) reaction center subunits, In this study , we investigated the effect of UV-B irradiation on the transcription of the psbA2 and psbA3 genes encoding identical D1 proteins. We show t hat UV-B irradiation increases the level of psbA2 mRNA 2-3-fold and, m ore dramatically, it induces a 20-30-fold increase in the accumulation of the psbA3 mRNA even at levels of irradiation too low to produce lo sses of either photosystem II activity or D1 protein. The induction of psbA3 transcript accumulation is specific for UV-B light (290-330 nm) . Low intensity UV-A emission (330-390 nm) and white light induce only a small, at most, 2-3-fold enhancement, whereas no effect of blue lig ht was observed, Expression patterns of chimeric genes containing the promoter regions of the psbA2, psbA3 genes fused to the firefly lucife rase (luc) reporter gene indicate that (i) transcription of psbA2/luc and psbA3/luc transgenes was elevated, similarly to that of the endoge nous psbA genes, by UV-B irradiation, and that (ii) a short, 80-base p air psbA3 promoter fragment is sufficient to maintain UV-B-induced tra nscription of the luc reporter gene. Furthermore, our findings indicat e that UV-B induced expression of the psbA2 and psbA3 genes is a defen se response against UV-B stress, which is regulated, at least, partial ly at the level of transcription and does not require active electron transport.