One SNV upstream of hemJ is conserved in all suppressor clones. Upper panel: The gene slr1790 encoding HemJ is transcribed from a single promoter with a transcription start site (arrow, +1) previously mapped to position 256,609 (NCBI reference sequence accession NC_000911.1) of the forward strand and associated with a 5ʹ UTR of 89 nt. Shown is the accumulation of sequencing reads in the genome-wide mapping of transcription start sites by differential RNA-seq.³⁴ The sharp decline in the sequence coverage results from the obtained average sequencing read length of 100 nt.³⁴ Lower panel: Resequencing of the genomes of seven suppressor clones revealed one conserved SNV at position 256,605 of the forward strand (position −4 with regard to the transcription start) changing C to T. This transition changes the GGCC methylation motif to GGTC (letter T above the sequence T) and is located between the promoter −10 element and the transcription start site, that is, within the promoter discriminator region (highlighted by the ochre background). Bisulfite sequencing data supporting the methylation of the slr1790 promoter GGCC motif in the WT are shown in Supplementary Fig. S1. A similar genetic arrangement exists in the related strain Synechocystis 6714 (Supplementary Fig. S8).