Redox control of psbA expression in cyanobacteria Synechocystis strains

The D1 reaction-centre protein of the Photosystem II complex is very sensit ive to Light. It is continuously damaged, degraded and synthesized. The res pective rates of these three processes are regulated by the light intensity . The means by which light regulates the expression of the psbA gene encodi ng the D1 protein in cyanobacteria is still an open question. Our results d emonstrate that photosynthetic electron transport has an important role in psbA expression in Synechocystis cells. Under steady illumination, addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or 2,5-dibromo-3-methyl- 6-isopropyl-benzoquinone (DBMIB) induces a transient activation of psbA tra nscription. Transcription of other photosynthetic genes like psaE and cpcBA , respectively encoding the PSA-E subunit of Photosystem I and the beta and alpha subunits of phycocyanin, one of the components of the phycobilisome, decreases under the same experimental conditions. Prolonged incubation wit h DCMU (or DCMU + methyl-viologen) results in a progressive decrease of psb A transcription and an increased stability of the transcript. Our data poin t to a control mechanism that involves two different signals: accumulation of Q(A)(-) specifically activates psbA transcription, whereas oxidation of the electron transfer chain downstream of photosystem II, most probably the plastoquinone pool and/or the cyt b(6)f, decreases the expression of psbA and that of other photosynthetic genes like psaE and cpcBA. (C) 1999 Elsevi er Science S.A. All rights reserved.