Expression of psbA genes encoding the photosystem II (PSII) reaction centre protein D1, was studied in the cyanobacterium Synechococcus sp. PCC 7942. Three psbA genes in Synechococcus encode two different forms of D1 protein, form I (D1:1) and form II (D1:2). The objective of this study was also to identify the role of these two forms of the D1 protein by studying expression of psbA genes under different environmental conditions. It was shown that a shift of cells from growth light to high light or a shift to lower temperature under constant light induced a rapid exchange of psbAI messages to psbAII/III messages. This change in the psbA transcript pool, in turn, was followed by a change of D1:1 to D1:2. psbA gene expression and high-light tolerance of PSII with different forms of DI protein was further studied by constructing the psbAI and psbAIII over-expression mutants. A strong hybrid promoter (tac-promoter) was cloned in front of either the psbAI or psbAIII gene to guarantee regulated expression of these genes upon addition of the inducer, EPTG, to the growth medium. It was found that PSII with D1:1 protein had a lower tolerance to high light than PSII with D1:2 protein. Further, the over-produced D1:2 protein was effectively inserted into PSII both under growth-light and high-light conditions, replacing the "low-light form", D1:1. Rapid degradation of D1:1 protein under such conditions was dependent on protein synthesis, suggesting that de novo synthesis of a protease is required for rapid D1:1/D1:2 exchange. psba over-expression experiments produced an interesting result: the mere presence of tac-psbA in Synechococcus DNA modified the expression of the endogenous psbA genes. Based on these observations, it was postulated that not only the 5' leader region, but also the beginning of the psbA gene coding region, might be involved in the regulation of psbA gene transcription. Indeed, a protein factor was found to bind to the 5' end of psbA genes (first 66 bp of the psbA gene coding region). Further, a relationship was found between the disappearance of the binding activity of this protein factor and the disappearance of psba messages after a shift of Synechococcus cells to very low light or darkness. Upon a shift back to growth light, the presence of a protein synthesis inhibitor, lincomycin, prevented rebinding of the protein factor and induction of the psbAI messages, but not of the psbAII/III messages. Thus, the expression of psbAI gene requires de novo synthesis of protein factors. Transcription of psbAII genes seems to be redox-regulated via a so far unknown electron transfer component. High transcriptional activity produced 5' psbA mRNA fragments. The 5' 320 hp mRNA fragment was demonstrated to be a degradation product of psbAI messages, since the complementary 3' end psbAI fragment was also found. The other, 5' 220 bp mRNA fragment was concluded to be a truncated psbA transcript originating from transcription of all psba genes. The production of truncated psbA transcripts was postulated to indicate down-regulation of psba transcription elongation, under conditions where more psba transcripts were produced than translated.