title: Organization of the BcgI restriction-modification protein for the transfer of one methyl group to DNA.
abstract: 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.
authors.affiliation: The DNA-protein Interactions Unit, School of Biochemistry, University of Bristol, University Walk, Bristol BS8 1TD, UK.
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Smith
authors.forename: Alistair J
authors.initials: AJ
authors.lastname: Jacklin
authors.forename: Jacqueline J T
authors.initials: JJ
authors.lastname: Marshall
authors.forename: Frank
authors.initials: F
authors.lastname: Sobott
authors.forename: Stephen E
authors.initials: SE
authors.lastname: Halford
mesh_terms: D019175:DNA Methylation; D009838:Oligodeoxyribonucleotides / Q000378:metabolism; D021122:Protein Subunits / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism; D015265:Site-Specific DNA-Methyltransferase (Adenine-Specific) / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D009838:Oligodeoxyribonucleotides; D021122:Protein Subunits; D012436:S-Adenosylmethionine; C085188:BcgI methyltransferase; D015265:Site-Specific DNA-Methyltransferase (Adenine-Specific)