In distinct, class transfers the methyl group to produce

In prokaryotes, DNA methylation is the only known
mechanism for epigenetic inheritance. The methylation reaction is catalyzed by
DNA methyltransferases (DNA MTases) in presence of a methyl donor (S-Adenosyl
–L- Methionine). In bacteria, DNA methylation is most often associated with
restriction modification systems which protects them against
bacteriophages.  Three functional classes
of MTases have been identified in bacteria and archaea. Two of these transfer a
methyl group from SAM to the duplex DNA, yielding N6-methyladenine
and N4-methylcytosine. A third, and mechanistically distinct, class
transfers the methyl group to produce 5-methylcytosine. However, in prokaryotes N6-MdA is the
best-characterized epigenetic regulator of gene expression (Kumar et al., 1994;
Malone, Blumenthal, & Cheng, 1995;
Shell et al., 2013).

Among the group of methyltransferases, Dam methyltransferase and CcrM has been found to regulate gene
expression in Gammaproteobacteria and Alphaproteobacteria respectively (Broadbent,
Balbontin, Casadesus, Marinus, & van der Woude, 2007; Casadesus & Low, 2006).
There are three m6A motifs and three corresponding m6A DNA MTase genes encoded
within the MTBC genomes, including the reported mamA (Rv3263), the hsdM
(Rv2756c) (8) and a newly discovered MTase gene (Rv2024c), which is  termed as mamB (Mycobacterial adenine
methyltransferase B). Of all these MamA Methyl Transferase has
been well characterized. A
methyltransferase, MamA (M.MtuHIII according to systematic DNA
methyltransferase nomenclature), and it is shown that it methylates a six base
pair sequence (CTGGAG) in the M. tuberculosis genome in a
strain specific manner .These MamA recognition site is predicted to be present
in 1947 locations in the H37Rv genome. Where, out of the 1816 times that MamA
sites occur within the coding regions, the six base pair sequence is located on
the coding strand in 1511 cases, while it is located on the non-coding strand
in only 305 cases. Any further changes to this six base pair sequence abrogated
methylation (Shell et al., 2013). MamA
is the predominant DNA methyltransferase in M. tuberculosis,
H37Rv. MamA is predicted to be a
Type II DNA methyltransferase with architecture of the gamma subtype (Wilson, 1992) , and is therefore homologous to the
well-characterized methyltransferases M.EcoKI and M.TaqI.  

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