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#Dam methylase

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Replication in Prokaryotes

Dam methylase methylates which sequence?

DNA adenine methyltransferase Dam transfers methyl groups to N6 position of adenine within palindromic sequence GATC widespread in E. coli genome occurring roughly every 256 base pairs randomly. After semiconservative replication, GATC becomes hemimethylated until Dam restores full methylation using S-adenosylmethionine. Transient state distinguishes nascent strand for mismatch repair strand discrimination and participates in origin sequestration via SeqA binding. Methylation does not affect base pairing but influences protein-DNA interactions altering binding affinities. Mutations in dam elevate mutations and asynchronous initiation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Dam methylase specificity for GATC and hemimethylation signal

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RDT Basics

Which methylase modifies adenine in GATC?

Escherichia coli possesses two major methyltransferases for restriction-modification protection. Dam methylase, deoxyadenosine methylase, recognizes palindromic 5'-GATC-3' and transfers a methyl group to N6 of adenine on both strands, influencing replication timing, mismatch repair, and gene regulation. Dcm methylase modifies internal cytosine in 5'-CCWGG-3' sequences. Methylation of GATC renders DNA resistant to cleavage by MboI which cuts only unmethylated sites, while DpnI requires Dam-methylated adenine for cutting. Understanding Dam methylation is essential for plasmid preparation, cloning, transformation, and restriction analysis protocols in molecular biology.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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