Cytochrome c_Mdownscales photosynthesis under photomixotrophy inSynechocystissp. PCC 6803

Abstract

CytM reduces photosynthesis under photomixotrophy One sentence summaryThe cryptic, highly conserved cytochrome c M completely blocks photosynthesis in Synechocystis under three days of photomixotrophy, possibly by suppressing CO 2 assimilation. AbstractPhotomixotrophy 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 ctype heme protein widespread in cyanobacteria.Δ CytM 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 Δ CytM, 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.the intertwined photosynthetic and respiratory electron transport chain, in order to accommodate this new energy source. Results Deletion of CytM confers growth advantage onΔ CytM and Δ Cox/Cyd/CytM in photomixotrophy In order to elucidate the physiological role of CytM and its possible functional association with thylakoid-localised RTOs, we studied the Δ CytM, Δ Cox/Cyd and Δ Cox/Cyd/CytM mutants. Unmarked mutants of Synechocystis lacking CytM were constructed by disrupting the cytM gene (sll1245) in WT (Supplemental Fig. S2) and the Δ CytM nor Δ Cox/Cyd/CytM demonstrated a growth advantage compared to WT and Δ Cox/Cyd, respectively. Under LAHG condition, Δ CytM grew faster than WT as previously reported (Hiraide et al 2015). The Δ Cox/Cyd and Δ Cox/Cyd/CytM mutants were unable to grow under LAHG. Previously it was reported that Cox is indispensable under this condition (Pils et al 1997).

Photosynthetically produced sucrose by immobilized Synechocystis sp. PCC 6803 drives biotransformation in E. coli

“…In line with the real-time gas exchange results the effective photosynthetic yield Y(II) in both cultures dropped from 0.37 to 0.08, which was accompanied by the decline of sucrose production. A possible reason for the decline in photosynthetic activity is the accumulation of sucrose, which switches the cell metabolism into photomixotrophy, when the cells start metabolizing sucrose as a carbon source while performing photosynthesis and CO 2 fixation [30]. This form of metabolism, which provides phototrophic cells with extra energy and carbon, often occurs when organic carbon sources are available in the environment, for example during phytoplankton blooms.…”

Engineering a Central Carbon Metabolism Pathway to Increase the Intracellular Acetyl-CoA Pool in Synechocystis sp. PCC 6803 Grown under Photomixotrophic Conditions

“…Photomixotrophic growth is a mode and metabolic state that photosynthetic microorganisms can perform photosynthesis and heterotrophic metabolism of utilizing organic carbon substrates simultaneously . Instead of growth solely dependent on light and CO 2 under photoautotrophic condition, photomixotrophic growth provides extra sources of energy and carbon and allows for cell growth and chemical production from both reduced carbon feedstocks and CO 2 .…”