Methy lase

Among the acceptors modified by S-adenosylmethionine are specific bases in DNA. The methylation of DNA protects bacterial DNA from cleavage by restriction enzymes (Section 9.d). d he base to be methylated is flipped out of the DNA double helix into the active site of a DNA methy-lase where it can accept a methyl group from S =adenosylmethionine (Figure 24.12). A recurring S-adenosylmethionine-binding domain is present in many SAM-dependent methylases. 

The introduction of a methyl group (CH3- or -CH3) into the nucleotide base residues, for example, into adenine and cytosine, will interrupt eukaryotic gene expression not only in bacteria but in mammals also (Voet and Voet, 1995, p. 1065ff). How this is done is not yet known, but it is presumably catalyzed by enzymes called methy-lases, with the methyl group furnished by some methyl donor such as 5-adenos-inemethionine. The degree of DNA eukaryotic methylation varies with species. 

Skinner, R., Cundliffe, E., and Schmidt, F. J.-(1983). Site of action of a ribosomal RNA methy-lase responsive to erythromycin and other antibiotics. J. Biol. Chem. 258, 12702-17206. 

This additional blunting is required to repair any damage caused by coRI methy-lase. We have noticed variable amounts of nuclease activity in methylases and think this extra step is prudent. 

Two types of restriction-modification system have been found in bacteria. In type I systems, the methy-lase and R. e. are both associated with a complex containing three different polypeptide chains an a-chain with R.e activity, a P-chain with methylase activity and a y-chain with the recognition site for the DNA sequence. Type I systems require S-adenosyl-t-me-thionine and ATP for both R.e. and methylase activities they are less specific, and cleavage sites may be random and far removed (1,000 base pairs) from the 5 side of the recognition site. In type II systems, me-thylases and R.e. are separate, 5-adenosyl-L-methio-... 

A number of enzymes which bind polyanions such as DNA may be separated broadly into those which recognize special sequences with higher affinity than "bulk DNA" and those which bind nonspecifically. The first group includes RNA polymerases, repressors, cAMP receptor protein (CRP), the rho factor, restriction endonucleases, and methy-lases the second group includes histones and DNA-unwinding proteins. 

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