OVER-PRODUCTION OF THE D1 PROTEIN OF PHOTOSYSTEM-II REACTION-CENTER IN THE CYANOBACTERIUM SYNECHOCOCCUS SP PCC-7942

The unicellular cyanobacterium Synechococcus sp. PCC 7942 has three ps bA genes encoding two different forms of the photosystem II reaction c entre protein D1 (D1:1 and D1:2). The level of expression of these psb A genes and the synthesis of D1:1 and D1:2 are strongly regulated unde r varying light conditions. In order to better understand the regulato ry mechanisms underlying these processes, we have constructed a strain of Synechococcus sp. PCC 7942 capable of over-producing psbA mRNA and D1 protein. In this study, we describe the over-expression of D1:1 us ing a tac-hybrid promoter in front of the psbAI gene in combination wi th lacI(Q) repressor system. Over-production of D1:1 was induced by gr owing cells for 12 h at 50 mu mol photons m(-2) s(-1) in the presence of 40 or 80 mu g/ml IPTG. The amount of psbAI mRNA and that of D1:1 pr otein in cells grown with IPTG was three times and two times higher, r espectively. A higher concentration of IPTG (i.e., 150 mu g/ml) did no t further increase the production of the psbAI message or D1:1. The ov er-production of D1:1 caused a decrease in the level of D1:2 synthesis ed, resulting in most PSII reaction centres containing D1:1. However, the over-production of D1:1 had no effect on the pigment composition ( chlorophyll a or phycocyanin/number of cells) or the light-saturated r ate of photosynthesis. This and the fact that the total amounts of D1 and D2 proteins were not affected by IPTG suggest that the number of P SII centres within the membranes remained unchanged. From these result s, we conclude that expression of psbAI can be regulated by using the tac promoter and lacI(Q) system. However, the accumulation of D1:1 pro tein into the membrane is regulated by the number of PSII centres.