Genetic maps and putative regulatory DNA sequences for RM systems containing R.PvuII orthologs. A. Three classes of RM systems containing R.PvuII orthologs. Top is PvuII itself, and systems at the right share its basic layout. In the middle is NsoJ138I, exemplifying systems with a fused C-REase protein. The systems at right share this layout, though in Asp and Gsp the MTase gene is in the opposite orientation. At the bottom is a streptococcal system (“Ssp”) that lacks a C gene, as does a system from Lachnospiraceae (“Lba”). B. Putative regulatory regions. The ATG at bottom right is the initiation codon for the C gene (except for Lba and Ssp, which have no C gene so the REase initiator is shown). The names of systems with fused C-REase genes are in bold; the subset from cyanobacteria are shaded (sequences shared by these three are also shaded). Most systems have the MTase gene divergently oriented from the C gene, and the initiator reverse complement is indicated in bold (CAT); the MTase initiator for Pvu is very close to that for the C gene (overlapping the downstream-most C box, and underlined). In the Asp, Gsp, Ssp and Lba systems the MTase and C (REase) are convergent. Rectangles indicate symmetry elements from C boxes (based on PvuII), with matches in other systems indicated in bold. Lba and Ssp, which lack a C gene and (presumably) C boxes, are shown for comparison. Species sources are: Pvu (Proteus vulgaris), Esp (Escherichia spp. TW09308), Spt (Salmonella enterica Paratyphi A), Sba (Salmonella enterica Bareilly), Pwa (Pectobacterium wasabei), Xsp (Xanthomonas spp.), Vei (Verminephrobacter eiseniae), Bce (Bacteroides cellulosilyticus), Mru (Meiothermus ruber), Nso (Niabella soli), Oni (Oscillatoria nigro-viridis), Asp (Anabaena spp.), Gsp (Gloeocapsa spp.), Lba (Lachnospiraceae bacterium), and Ssp (Streptococcus spp. GMD1S). GenBank accession numbers are in Methods.