The SsoII and NlaX DNA methyltransferases: Overproduction and functional analysis

Overproduction of the NlaX DNA methyltransferase (M· NlaX) in an Escherichia coli host conferred resistance to SsoII restriction endonuclease (R· SsoII) digestion. This suggested an overlap of sequence specificity between M· NlaX and M· SsoII, the latter of which modifies the internal cytosine of the target sequence 5′-CCNGG-3′. A variant of M· NlaX (M· Sso/Nla), containing an N-terminal extension from M· SsoII, was also enzymatically active. Using deletion analysis, the N-terminal 71 amino-acid residues of M· SsoII were shown to be essential for modification activity.