Type III restriction/modification enzyme recognize short, non-palindromic sequences that can be methylated on only one strand, with the paradoxical consequence that during replication of what is in effect hemimethylated DNA totally unmodified sites arise. Why the unmodified sites are not subject to suicidal restriction was not clear. Here we show that restriction requires two unmodified recognition sites that can be separated by different distances but which must be in inverse orientation. All of the unmodified sites in newly replicated DNA are of course in the same orientation, which explains why they are not restricted. This result may be of relevance to other manifestations of anisotropy in double-stranded DNA, such as genetic imprinting.
A nomenclature is described for restriction endonucleases, DNA methyltransferases, homing endonucleases and related genes and gene products. It provides explicit categories for the many different Type II enzymes now identi®ed and provides a system for naming the putative genes found by sequence analysis of microbial genomes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.