Evolution and function of the neisserialdam-replacing gene

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Abstract
Phase variation through slippage‐like mechanisms involving homopolymeric tracts depends in part on the absence of Dam‐methylase in several pathogenic isolates of Neisseria meningitidis. In Dam‐defective strains drg (dam‐replacing gene), flanked by pseudo‐transposable small repeated elements (SREs), replaced dam. We demonstrate that drg encodes a restriction endonuclease (NmeBII) that cleaves 5′‐GmeATC‐3′. drg is also present in 50% of Neisseria lactamica 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 GATmeC, suggesting an alternative restriction‐modification system (RM) specific for 5′‐GATC‐3′, similar to Sau3AI‐RM of Staphylococcus aureus 3A, Lactococcus lactis KR2 and Listeria monocytogenes.