Methylases

INTRODUCTION, TYPES, GENES ,dam gene,dcm gene, E.coli methylases, USES

1. METHYLASES M.Meenakshi , Assistant Professor, Department of Microbiology, Sri Ramakrishna CAS for Women, Coimbatore

2. METHYLASES Enzymes called methylases add methyl groups (—CH 3 ) to adenine or cytosine bases within the recognition sequence, which is thus modified and protected from the endonuclease. The restriction enzyme and its corresponding methylase constitute the restriction-modification system of a bacterial species.

3. METHYLASES

4. TYPES OF METHYLASES

5. METHYLASES   The bacterial restriction-modification system has provided a variety of specific endonucleases, there are also available a variety of specific DNA methylases . The recognition sequences of the methylases are the same as the associated endonucleases (e.g. EcoR1 methylase recognizes and methylates at the sequence "GAATTC"). 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. Methylation of DNA usually has the effect of protecting the DNA from the related restriction endonuclease. However , there are methylases with minimal specificity. For example,  Sss I methylase  will methylate cytosine residues in the sequence 5' … CG … 3'. In this case,  the methylated DNA will be protected from a wide variety of restriction endonucleases .

6. GENES INVOLVED IN METHYLASES The methylase encoded by the  dam   gene   (DNA adenine  methyltransferase   enzyme)   transfers a methyl group from SAM to the N 6  position of the adenine base in the sequence 5' … GATC … 3'. The methylase encoded by the  dcm  gene ( DNA-cytosine methyltransferase ) methylates the internal cytosine base, at the C 5  position, in the sequences 5' … CCAGG … 3' and 5' … CCTGG … 3'. The methylases are of interest here for two reasons . First, some or all of the sites for a restriction endonuclease may be resistant to cleavage when isolated from strains expressing the Dam or Dcm methylases . This occurs because DNA is protected from cleavage when a particular base in the recognition site of a restriction endonuclease is methylated . The relevant base may be methylated by one of the  E. coli   methylases

7. METHYLASES IN E.COLI

8. USES OF METHYLASES In prokaryotes, the major role of DNA methylation is to protect host DNA against degradation by restriction enzymes . In eukaryotes, DNA methylation has been implicated in the control of several cellular processes, including differentiation, gene regulation, and embryonic development Used in recycling of heavy metals like arsenic, mercury, cadmium. Histone methyltransferases, Histones that are methylated on certain residues can act epigenetically to repress or activate gene expression . [ Protein methylation is one type of post-translational modification Global methylation in species ranging from human, mouse, fish, fly, round worm, plant, algae and cyanobacteria causes the same effects on their biological rhythms, demonstrating conserved physiological roles of methylation during evolution

9. REFERENCES Walsh, Christopher (2006).  "Chapter 5 – Protein Methylation"  (PDF).  Posttranslational modification of proteins: expanding nature's inventory . Roberts and Co. Publishers.  ISBN   978-0-9747077-3- https://bio.libretexts.org/Bookshelves/Biochemistry/Supplemental_Modules_(Biochemistry)/1%3A_DNA/1.4%3A_DNA_Modifying_Enzymes

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