THE EFFECT OF PHOTOSYSTEM-II INHIBITORS DCMU AND BNT ON THE HIGH-LIGHT INDUCED D1 TURNOVER IN 2 CYANOBACTERIAL STRAINS SYNECHOCYSTIS PCC-6803 AND SYNECHOCOCCUS PCC-7942

The effect of the Photosystem II (PSII) inhibitors dichlorophenyldimet hylurea (DCMU) and bromonitrothymol (BNT) on the rate of the high-ligh t induced D1 protein turnover was studied in whole cells of two cyanob acterial strains Synechocystis PCC 6803 and Synechococcus PCC 7942. In Synechocystis the D1 degradation was slowed down to a similar extent in the presence of either inhibitor compared with control cells. This slower degradation corresponded with the retardation of Photosystem II photoinactivation (PSIIPI) measured as a decline of PS II activity in the illuminated cells treated with chloramphenicol (CAP). The ongoing DI synthesis in the presence of both PS II inhibitors was confirmed b y unchanging PS II activity and the steady-state level of D1 during il lumination in the absence of CAP. In Synechococcus cells both DCMU and BNT blocked the turnover of the 'low-light' D1 form (D1:1) but did no t prevent the exchange of the 'high-light' form D1:2 for the D1:1 form . The similar effect of both herbicides on the D1 exchange was in cont rast with their influence on the rate of PSIIPI. While DCMU had a pron ounced protective effect, BNT significantly increased the rate of PS I I photodamage. The fast BNT-induced decline of PS II activity was also observed in Synechocystis cells treated with azide, an inhibitor of r eactive oxygen species scavenging enzymes. Therefore, we assume that t he distinct sensitivity of the two cyanobacterial strains to BNT can b e caused by different content and/or activity of these enzymes in each strain.