Optical study of cytochrome cM formation in Synechocystis

The expression of the cytM gene, which encodes cytochrome c(M) in Synechocystis sp. PCC 6803, is induced under stress conditions such as low temperature. Results of spectrophotometric studies revealed that cytochrome c(M) was oxidized by light absorbed by photosystem I in cells that had acclimatized to a low temperature. However, no similar light-induced oxidation of cytochrome c(M) was observed in Delta cytM mutant cells. The kinetics of the oxidation and reduction of P700 before and after acclimation of Synechocystis cells to low temperature suggested that cytochrome c(M) might donate electrons to photooxidized P700. Our observations indicate that cytochrome c(M) is synthesized under low-temperature stress and that it might carry electrons directly to P700(+) in photosystem I.