A nucleotide insertion between two adjacent methyltransferases in Helicobacter pylori results in a bifunctional DNA methyltransferase

Helicobacter pylori has a dynamic R-M (restriction–modification) system. It is capable of acquiring new R-M systems from the environment in the form of DNA released from other bacteria or other H. pylori strains. Random mutations in R-M genes can result in non-functional R-M systems or R-M systems with new properties. hpyAVIAM and hpyAVIBM are two solitary DNA MTase (methyltransferase) genes adjacent to each other and lacking a cognate restriction enzyme gene in H. pylori strain 26695. Interestingly, in an Indian strain D27, hpyAVIAM–hpyAVIBM encodes a single bifunctional polypeptide due to insertion of a nucleotide just before the stop codon of hpyAVIBM and, when a similar mutation was made in hpyAVIAM–hpyAVIBM from strain 26695, a functional MTase with an N-terminal C⁵-cytosine MTase domain and a C-terminal N⁶-adenine MTase domain was constructed. Mutations in the AdoMet (S-adenosylmethionine)-binding motif or in the catalytic motif of M.HpyAVIA or M.HpyAVIB selectively abrogated the C⁵-cytosine or N⁶-adenine methylation activity of M.HpyAVIA–M.HpyAVIB fusion protein. The present study highlights the ability of H. pylori to evolve genes with unique functions and thus generate variability. For organisms such as H. pylori, which have a small genome, these adaptations could be important for their survival in the hostile host environment.