Regulation of the Stoichiometry of Thylakoid Components in the Photosynthetic System of Cyanophytes: Model Experiments Showing that Control of the Synthesis or Supply of Chl a can Change the Stoichiometric Relationship between the Two Photosystems

Regulation of the assembly of the photosystem I (PS I) complexin response to the light regime in the photosynthetic systemof cyanophytes was studied in Synechocystis PCC 6714. The relationshipbetween the assembly of the PS I complex and synthesis of Chla was examined by model experiments in which synthesis of Chla was controlled by two inhibitors, gabaculine (GAB) and 2,2'-dipyridyl(DP). Both inhibitors caused a change to a lower ratio of PSI to PS II even under light that normally induces a high ratioof PS I to PS II. The change in stoichiometry induced by theseinhibitors was suppressed when protein synthesis was inhibitedby chloram-phenicol, similarly to the change in the stoichiometryinduced by light that excites mainly PS I (PS I light). Comparisonof the levels of PS I, PS II and Cyt b₆-f complexes per cellindicated that a selective suppression of the assembly of thePS I complex was induced by the inhibitors: the stoichiometricrelationship among PS I, PS II and Cyt b₆-f complexes was identicalto that induced by PS I light or white light of high intensity.GAB induced a decrease in size of the phycobilisome also, whileDP did not, similarly to PS I light. The results indicate thatthe ratio of PS I to PS II can be changed by the control ofsynthesis of Chl a. They also suggest that control of the synthesisor supply of Chl a probably exerted at site(s) in or after theprocess of the Mg-protoporphyrin branch, is involved in themechanism of regulation of the assembly of the PS I complexin cyanophytes. (Received September 7, 1989; Accepted November 20, 1989)