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Thymine derived from m ethyl cytosine is removed by the "very short patch repair" system.

catch a T-G mismatch if generated as a replication error and found in newly made DNA. However, deamination occurs spontaneously at any time and rarely during replication. Consequently, deamination of 5-methylcytosine often goes un-repaired and the presence of 5-methylcytosine leads to mutational hot-spots as discussed in Chapter 13.

Nonetheless, most of the 5-methylcytosine in E. coli is made by Dcm methylase and is found in the sequences CCAGG and CCTGG. Whenever T is found replacing C and therefore paired with G in one of these sequences, it is removed. A specific endonuclease nicks the DNA next to the T of the T-G mismatch. This system is sometimes known as "very short patch repair" and the nicking enzyme as Vsr endonuclease. DNA polymerase I then removes a short length of the strand with the incorrect T and replaces it with a new piece of DNA. Although methylating the cytosine in

FIGURE 14.19 Suicide Demethylase for O-Methyl Bases

Demethylase protein recognizes CH3 attached via oxygen (O) to either guanine or thymine. The demethylase transfers the CH3 to itself then degrades itself. The correct base structure is restored by removal of the methyl group without any other modification.

Methyl transfer from base to protein

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