Epigenetic DNA modifications are pivotal in eukaryotic gene expression, but their regulatory significance in bacteria is less understood. InSynechocystis6803, the DNA methyltransferase M.Ssp6803II modifies the first cytosine in the GGCC motif, forming N4-methylcytosine (GGm4CC). Deleting thesll0729gene (Δsll0729) caused a bluish phenotype due to reduced chlorophyll levels, which was reversed by suppressor mutations. Re-sequencing of seven suppressor clones revealed a common GGCC to GGTC mutation in theslr1790promoter’s discriminator sequence, encoding protoporphyrinogen IX oxidase, HemJ, crucial for tetrapyrrole biosynthesis. Transcriptomic and qPCR analyses indicated aberrantslr1790expression in Δsll0729mutants. This aberration led to the accumulation of coproporphyrin III and protoporphyrin IX, indicative of impaired HemJ activity. To confirm the importance of DNA methylation inhemJexpression, native and mutatedhemJpromoter variants were introduced into the wild type, followed bysll0729deletion. Thesll0729deletion segregated in strains with the GGTC motif in thehemJpromoter, resulting in wild-type-like pigmentation, whereas freshly prepared Δsll0729mutants with the nativehemJpromoter exhibited the bluish phenotype. These findings demonstrate thathemJis tightly regulated inSynechocystisand that N4-methylcytosine is essential for properhemJexpression.