Regulation of synthesis of PSI in the cyanophytes Synechocystis PCC6714 and Plectonema boryanum during the acclimation of the photosystem stoichiometry to the light quality

The effects of light quality on the formation of the PSI complex were examined in Synechocystis PCC6714 and in Plectonema boryanum. The rate of increase in levels of core polypeptides of PSI, PsaA/B, doubled upon shift from Chl a-absorbed light (PSI light) to phycobilisome-absorbed light (PSII light). The elevated rate was decreased upon the reverse shift. Half time of the acceleration was approximately 10 min, and that of the decrease was approximately 4 min. The rate of degradation of the polypeptides was far lower than the rate of the increase under either light regime. Neither synthesis nor degradation of the PsbA and PsbC polypeptides of PSII was significantly altered by the light quality. We conclude that synthesis of the PSI complex is chromatically regulated to allow adjustments in photosystem stoichiometry. The level of mRNA for PsaA/B was not altered by the light regime. Anomalous inhibition by chloramphenicol suggested that the regulation occurs at a step(s) other than the peptide elongation step, perhaps at the initiation of the ribosome cycle or at the insertion of Chl a for the stabilization of the polypeptides. The photoreduction of protochlorophyllide (Pchlide) was compared with the synthesis of the polypeptides in a mutant of Plectonema boryanum that lacked Pchlide dark reductase (YFC1004). The results indicated that the synthesis of stable PsaA/B polypeptides was not limited by the reduction of Pchlide, although the synthesis did depend on a supply of Chl a.