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. 2012 Sep 23;40(21):10916–10924. doi: 10.1093/nar/gks876

Figure 1.

Figure 1.

Structures of Type I MTases showing the HsdS subunit with TRDS (yellow) and conserved helical regions (orange), and the two HsdM subunits (one in cyan, the other with the N-terminal domain in green, the MTase catalytic domain in blue and the C-terminal helical tail in grey). (a) The EcoKI MTase structural model (14). The C terminus of an M subunit and the N terminus of the S subunit are indicated by ‘C’ and ‘N’, respectively, and are close together in space. (b) A schematic of the gene layout for typical Type IA/IC RM systems and for Type IB RM systems. The frameshift at the junction between regions coding for HsdM and HsdS subunits is indicated by showing the hsdM gene above the hsdS gene. The colour scheme is as in part (a) (with the additional inserts in the hsdS gene for the IB system coloured grey for reasons discussed later). A small conserved section at the start of hsdS is not shown for clarity. (c) The circular arrangement of HsdS subunit sequences for a Type IA/IC HsdS subunit, as originally proposed by Kneale (23), for a half-S subunit and for the Type IB HsdS subunit.