Cytochrome cMdownscales photosynthesis under photomixotrophy inSynechocystissp. PCC 6803

Photomixotrophy is a metabolic state, which enables photosynthetic microorganisms to simultaneously perform photosynthesis and metabolism of imported organic carbon substrates. This process is complicated in cyanobacteria, since many, including Synechocystis sp. PCC 6803, conduct photosynthesis and respiration in an interlinked thylakoid membrane electron transport chain. Under photomixotrophy, the cell must therefore tightly regulate electron fluxes from photosynthetic and respiratory complexes. In this study, we show via characterization of photosynthetic apparatus and the proteome, that photomixotrophic growth results in a gradual reduction of the plastoquinone pool in wild-type Synechocystis , which fully downscales photosynthesis over three days of growth. This process is circumvented by deleting the gene encoding cytochrome c M (CytM), a cryptic c -type heme protein widespread in cyanobacteria. ΔCytM maintained active photosynthesis over the three day period, demonstrated by high photosynthetic O 2 and CO 2 fluxes and effective yields of Photosystem II and Photosystem I. Overall, this resulted in a higher growth rate than wild-type, which was maintained by accumulation of proteins involved in phosphate and metal uptake, and cofactor biosynthetic enzymes. While the exact role of CytM has not been determined, a mutant deficient in the thylakoid-localised respiratory terminal oxidases and CytM (ΔCox/Cyd/CytM) displayed a similar phenotype under photomixotrophy to ΔCytM, demonstrating that CytM is not transferring electrons to these complexes, which has previously been suggested. In summary, the obtained data suggests that CytM may have a regulatory role in photomixotrophy by reducing the photosynthetic capacity of cells. One sentence summary The cryptic, highly conserved cytochrome c M completely blocks photosynthesis in Synechocystis under three days of photomixotrophy, possibly by suppressing CO 2 assimilation.