DNA-methylation of GGCC motifs via M.Ssp6803II is linked to DNA repair in the cyanobacterium Synechocystis sp. PCC 6803 [sll0729 mutant]

DNA methylation plays a crucial role for gene regulation among eukaryotes, but its regulatory function is less understood in bacteria. In the cyanobacterium Synechocystis sp. PCC 6803 five DNA methyltransferases have been identified. Among them, M.Ssp6803II is responsible for the specific methylation of the first cytosine in the frequently occurring motif GGCC, leading to N4-methylcytosine (GGm4CC). The mutation of the corresponding gene sll0729 led to a strong phenotype, including a lowered chlorophyll/phycocyanin ratio, impaired growth, and alterations of the expression of two genes encoding hypothetical proteins. However, prolonged cultivation revealed instability of the initially obtained phenotype, with colonies showing normal pigmentation and wild-type-like growth appearing regularly and in high frequencies on agar plates. These colonies represent suppressor mutants, since the sll0729 gene was still completely inactivated and the GGCC sites remained un-methylated. Two suppressor clones were further characterized and showed a smaller cell size and lowered DNA content per cell. Moreover, they exhibited a decreased tolerance against UV stress compared to wild type. Proteomics revealed the reduction of DNA topoisomerase in suppressor mutant cells. Collectively, these results indicate that GGm4CC methylation is involved in the regulation of gene expression, in the fine-tuning of DNA replication, and DNA repair mechanisms. We analyzed gene expression in various mutant strains of Synechocystis sp. PCC 6803 compared to WT.