E. coli restriction-modification system

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.