Organization of the BcgI restriction-modification protein for the transfer of one methyl group to DNA.

The Type IIB restriction-modification protein BcgI contains A and B subunits in a 2:1 ratio: A has the active sites for both endonuclease and methyltransferase functions while B recognizes the DNA. Like almost all Type IIB systems, BcgI needs two unmethylated sites for nuclease activity; it cuts both sites upstream and downstream of the recognition sequence, hydrolyzing eight phosphodiester bonds in a single synaptic complex. This complex may incorporate four A(2)B protomers to give the eight catalytic centres (one per A subunit) needed to cut all eight bonds. The BcgI recognition sequence contains one adenine in each strand that can be N-6-methylated. Although most DNA methyltransferases operate at both unmethylated and hemi-methylated sites, BcgI methyltransferase is only effective at hemi-methylated sites, where the nuclease component is inactive. Unlike the nuclease, the methyltransferase acts at solitary sites, functioning catalytically rather than stoichiometrically. Though it transfers one methyl group at a time, presumably through a single A subunit, BcgI methyltransferase can be activated by adding extra A subunits, either individually or as part of A(2)B protomers, which indicates that it requires an assembly containing at least two A(2)B units.