Optical study of cytochrome c(M) formation in Synechocystis

The expression of the cytM gene, which encodes cytochrome c(M) in Synechocy stis sp. PCC 6803, is induced under stress conditions such as low temperatu re. 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 cytoc hrome c(M) was observed in Delta cytM mutant cells. The kinetics of the oxi dation and reduction of P700 before and after acclimation of Synechocystis cells to low temperature suggested that cytochrome c(M) might donate electr ons to photooxidized P700. Our observations indicate that cytochrome c(M) i s synthesized under low-temperature stress and that it might carry electron s directly to P700(+) in photosystem I.