The slow S to M fluorescence rise is missing in the RpaC mutant of synechocystis sp. (PCC 6803)

Abstract

Transfer of phycobilisome-containing cyanobacteria from darkness to continuous light results in a typical chlorophyll a fluorescence induction that follows a pattern labeled as OJIPSMT. This pattern of fluorescence induction (FI) reflects changes in both photochemical and non-photochemical processes. We have focused on the slow S to M fluorescence rise that is dominant in cyanobacteria. We clearly observe the S-to-M fluorescence rise in the wild type (WT) cells of Synechocystis sp. (PCC 6803) in the presence of 1, 1′-dimethyl-3(3′4′-dichloro)-phenylurea (DCMU). This rise is fully suppressed by hyperosmotic glycine betaine that prevents the mobility of extramembrane phycobilisomes. The contribution of the State 2-to-State 1 transition in the S-to-M rise of WT cells was proven by changes in the 77 K emission spectra: The spectra at point O of FI (i.e., in State 2, with lower ratio of F685/F726) were characteristically different from those at point M (i.e., in State 1, with higher ratio of F685/F726). The S-M rise was totally missing in RpaC— mutant of Synechocystis sp. (PCC 6803) that is locked in the high-fluorescence State 1 and thus is unable to do state transitions. Moreover, RpaC-mutant showed quenching of phycobilin fluorescence during the S-M rise period. Taken together, these data suggest that the State 2 to State 1 transition is the dominant cause of the S to M fluorescence rise in cyanobacteria.