Nucleotide sequence of the Dpn II DNA methylase gene of Streptococcus pneumoniae and its relationship to the dam gene of Escherichia coli.

Abstract

The structural gene (dpnM) for the Dpn II DNA methylase of S. pneumoniae, which is part of the Dpn II restriction system and methylates adenine in the sequence 5''-G-A-T-C3'', was identified by subcloning fragments of a chromosomal segment from a Dpn II-producing strain in the S. pneumoniae host/vector cloning system and demonstrating function of the gene also in Bacillus subtilis. Determination of the nucleotide sequence of the gene and adjacent DNa indicates that it encodes a polypeptide of 32,903 daltons. A putative promoter for transcription of the gene lies within 100 nucleotides of the polypeptide start codon. Comparison of the coding sequence to that of the dam gene of E. coli, which encodes a similar methylase, revealed 30% of the amino acid residues in the 2 enzymes to be identical. This homology presumably reflects a common origin of the 2 genes prior to the divergence of gram-positive and gram-negative bacteria. Thus, the restruction function of the gene is primitive, and the homologous restriction system in E. coli has evolved to play an accessory role in heteroduplex DNA base mismatch repair.