Lumenal proteins involved in respiratory electron transport in the cyanobacterium Synechocystis sp. PCC6803.

Cyanobacterial thylakoids catalyze both photosynthetic and respiratory activities. In a photosystem I-less Synechocystis sp. PCC 6803 strain, electrons generated by photosystem II appear to be utilized by cytochrome oxidase. To identify the lumenal electron carriers (plastocyanin and/or cytochromes c553, c550, and possibly cM) that are involved in transfer of photosystem II-generated electrons to the terminal oxidase, deletion constructs for genes coding for these components were introduced into a photosystem I-less Synechocystis sp. PCC 6803 strain, and electron flow out of photosystem II was monitored in resulting strains through chlorophyll fluorescence yields. Loss of cytochrome c553 or plastocyanin, but not of cytochrome c550, decreased the rate of electron flow out of photosystem II. Surprisingly, cytochrome cM could not be deleted in a photosystem I-less background strain, and also a double-deletion mutant lacking both plastocyanin and cytochrome c553 could not be obtained. Cytochrome cM has some homology with the cytochrome c-binding regions of the cytochrome Caa3-type cytochrome oxidase from Bacillus spp. and Thermus thermophilus. We suggest that cytochrome cM is a component of cytochrome oxidase in cyanobacteria that serves as redox intermediate between soluble electron carriers and the cytochrome aa3 complex, and that either plastocyanin or cytochrome c553 can shuttle electrons from the cytochrome b6f complex to cytochrome cM.