The deficiency of methylglyoxal synthase promotes cell proliferation in <i>Synechocystis</i> sp. PCC 6803 under mixotrophic conditions

Methylglyoxal synthase (MGS), which converts dihydroxyacetone phosphate to methylglyoxal (MG), is found in only prokaryotes. <i>Synechocystis</i> sp. PCC 6803 possesses the gene <i>sll0036</i>, which encodes MGS. To clarify the biological function of MGS, we constructed a gene-disruption strain of <i>Synechocystis</i> sp. PCC 6803. Expression analysis showed that MG metabolic genes (<i>sll0036</i>, <i>sll0067</i>, and <i>slr1167</i>) were upregulated under photoautotrophic conditions compared to mixotrophic conditions. The <i>sll0036</i>-deficient strain (Δ0036) exhibited a higher growth rate than the wild-type (WT) strain under mixotrophic conditions, whereas no significant difference was observed under photoautotrophic conditions. When cells were cultured in a medium supplemented with sorbitol or mannitol instead of glucose, the growth enhancement observed in the Δ0036 strain disappeared. This suggests that the difference in growth between Δ0036 and WT is influenced by glucose-related metabolism rather than osmotic stress. MG contents were found to be decreased in the Δ0036 strain compared to WT under mixotrophic conditions. This suggests that the reduction of MG level might activate the cell proliferation of <i>Synechocystis</i> sp. PCC 6803 under mixotrophic conditions.