Figure 1.

Structural organization of the AhdI restriction-modification system. The ahdI genes are schematically shown on the top. Arrows show the direction of transcription. Two ahdI transcription units (MS and CR) are separated by a short self-complementary region that may act as a bidirectional transcription terminator (depicted here as a hairpin). DNA sequences around transcription start points of the ahdIMS and ahdICR promoters are expanded below. Beginnings of coding regions are indicated with colours that match those at the top of the figure. The PahdIMS and PahdICR start sites are shown, respectively, by rightward and leftward arrows above and below the sequence. Promoter elements are underlined and transcription starts are colour-coded. The ahdI C-box is indicated and two sets of inverted repeats are shown by convergent arrows. The AhdI site in front of PahdIMS is indicated in red; astericks indicate methylated adenine residues. The start codons of ahdC and ahdIM are indicated.