coli commonly used in cloning have a dam+ dcm+ genotype. The methylase encoded by the dcm gene (dcm methylase) methylates the internal cytosine base, at the C 5 position, in the sequences 5' … CCAGG … 3' and 5' … CCTGG … 3'.The methylase encoded by the dam gene (dam methylase) transfers a methyl group from SAM to the N 6 position of the adenine base in the sequence 5' … GATC … 3'.Still other restriction endonucleases will cut both methylated and non-methylated DNA at their recognition sequences (e.g.Some restriction endonucleases will only cut DNA at their recognition sites if the DNA is methylated (e.g.In this case, the methylated DNA will be protected from a wide variety of restriction endonucleases. For example, Sss I methylase will methylate cytosine residues in the sequence 5' … CG … 3'. However, there are methylases with minimal specificity. Methylation of DNA usually has the effect of protecting the DNA from the related restriction endonuclease.All methylases transfer the methyl group from S-adenosylmethionine (SAM) to a specific base in the recognition sequence, and SAM is a required component in the methylation reaction.EcoR1 methylase recognizes and methylates at the sequence "GAATTC"). The recognition sequences of the methylases are the same as the associated endonucleases (e.g.Just as the study of the bacterial restriction-modification system has provided a variety of specific endonucleases, there are also available a variety of specific DNA methylases.
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