This document discusses methylases, which are enzymes that add methyl groups to DNA. Specifically: - Methylases transfer methyl groups from S-adenosylmethionine to adenine or cytosine bases within their recognition sequence on DNA. This methylation protects the DNA from restriction endonucleases. - The methylase and restriction enzyme of a bacterial species together form the restriction-modification system, with the methylase protecting the host DNA. - Methylases are of interest because methylation of some restriction enzyme recognition sites protects the DNA from being cleaved by that enzyme. This allows study of DNA isolated from strains expressing common methylases like Dam or Dcm.

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

2. METHYLASES
 Enzymes called methylases
add methyl groups (—CH3)
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 N6 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 C5 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_D
NA_Modifying_Enzymes

10. THANK YOU