E. coli restriction-modification system

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Notes of relevance to me taken from Escherichia coli & Salmonella

  • Restriction systems are only found in unicellular organisms.
    • Either the cell modifies certain sequences so they are not restricted (classic R-M systems)
    • or certain foreign modifications are restricted.
  • Mcr Systems
    • McrBC - cleaves hm5C, m5C and m4C (hydroxymethylated or methylated cytosine at positions 4 or 5). The modified bases must be paired about 40-80bp apart. (genes mcrB and mcrC)
    • McrA - cleaves DNA mehylated by HpaII and SssI methylases. (encoded by the prophagelike e14 element)
  • Classic R-M systems

A classi R-M system includes an endonuclease that cleaves a specific DNA sequence and a DNA methyltransferase that methylates either adenosyl or cytosyl residues within the same DNA sequence.

    • Type 1 R-M systems
      • Relatively rare.
      • One, three-subunit protein acts as both endonuclease and methylase.
      • Requires AdoMet as a cofactor.