Bacteriophage-T4 UV endonuclease nicks the C(3′)-O-P bond 3′ to AP (apurinic or apyrimidinic) sites by a beta-elimination reaction. The breakage of this bond is sometimes followed by the nicking of the C(5′)-O-P bond 5′ to the AP site, leaving a 3′-phosphate end; delta-elimination is proposed as a mechanism to explain this second reaction. The AP site formed when this enzyme acts on a pyrimidine dimer in a polynucleotide chain undergoes the same nicking reactions. Micrococcus luteus UV endonuclease also nicks the C(3′)-O-P bond 3′ to AP sites by a beta-elimination reaction. No subsequent delta-elimination was observed, but this might be due to the presence of 2-mercaptoethanol in the enzyme preparation.

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