Evolution and function of the neisserial dam-replacing gene

Phase variation through slippage-like mechanisms involving homopolymeric tr acts depends in part on the absence of Dam-methylase in several pathogenic isolates of Neisseria meningitidis, In Dam-defective strains drg (dam-repla cing gene). flanked by pseudo-transposable small repeated elements (SREs). replaced dam. We demonstrate that drg encodes a restriction endonuclease (N meBII) that cleaves 5'-GmeATC-3'. drg is also present in 50% of Neisseria l actamica strains, but in most of them it is inactive because of the absence of an SRE-providing, promoter. This is associated with the presence of GAT meC suggesting an alternative restriction-modification system (RM) specific for 5'-GATC-3'. similar to Sau3AI-RM of Staphylococcus aureus 3A. Lactococ cus lactis KR2 and Listeria monocytogenes. 2001 Published by Elsevier Scien ce B.V. on behalf of the Federation of European Biochemical Societies.