High-resolution Crystal Structure of the Restriction–modification Controller Protein C.AhdI from Aeromonas hydrophila Original Research Article

Restriction–modification (R–M) systems serve to protect the host bacterium from invading bacteriophage. The multi-component system includes a methyltransferase, which recognizes and methylates a specific DNA sequence, and an endonuclease which recognises the same sequence and cleaves within or close to this site. The endonuclease will only cleave DNA that is unmethylated at the specific site, thus host DNA is protected while non-host DNA is cleaved. However, following DNA replication, expression of the endonuclease must be delayed until the host DNA is appropriately methylated. In many R–M systems, this regulation is achieved at the transcriptional level via the controller protein, or C-protein.