Cyanobacteria acclimate to changes in incident light by adjusting photosystem stoichiometry through regulation of PSI accumulation. To gain a deeper insight into this control mechanism in Synechocystis sp. strain PCC 6803, we studied the expression and regulation of the psaAB operon, encoding the reaction center proteins of PSI, during the initial stage of acclimation to changes in the intensity and quality of light. The psaAB operon was transcribed as a dicistronic transcript, which was processed into smaller, putatively monocistronic psaA and psaB transcript species. Dark treatment of the cells inhibited the psaAB transcription, whereas re-illumination of dark-adapted cells reactivated the transcription slowly in a process requiring de novo protein synthesis. Transfer of cells from white to orange light, favoring excitation of PSII, stimulated the psaAB transcription, whereas far-red light, primarily exciting PSI, down-regulated the transcription of the psaAB operon. These results, together with down-regulation of psaAB transcription upon the addition of electron transport inhibitors under constant white light illumination, suggested that the photosynthetic redox poise affects the psaAB transcription activity in the light. Pulse-labeling experiments demonstrated that light-induced modulations in the translation rate of the PsaA protein closely parallel the transcription rate of the psaAB operon, indicating that transcriptional regulation plays the major role in determining the content of PSI reaction center proteins and, thereby, PSI complexes, during light acclimation. The scantiness of PsaA translation in darkness despite the abundance of psaA transcripts demonstrated that the comprehensive regulation of PSI accumulation also involves regulation at the level of translation.