Schematic representation of the specificity
switching of the Type IIG MTase HpyUM032XIII. (A) The S1 and S2 subunits are separated by a homopolymeric tract of 12 G residues at the location shown by ▼, which appears to create a natural frameshift. (B) Reducing the tract length from 12 to 11 corrects the frameshift at ▼, thereby fusing the S1 and S2 subunits. This ‘corrected’ sequence is denoted by ‘mut’, to create fusion of S1 and S2 subunits. (C–D) Expression
of individual S subunits with the RM gene shows that the S1
subunit is active in the absence of S2.