The DNA polymerase beta reaction with ultraviolet-irradiated DNA incised by correndonuclease

Covalently closed circular Col E1 DNA was ultraviolet-irradiated with a dose of 60 J/m2, thus introducing about 3.2 pyrimidine dimers per DNA molecule. Treatment of irradiated Col E1 DNA with Micrococcus luteus correndonuclease resulted, in the vicinity of pyrimidine dimers, in an average of 3.3 incisions per DNA molecule, and converted DNA to the open circular form. Incised Col E1 DNA stimulated no reaction with calf thymus DNA polymerase alpha but was recognized as a template by DNA polymerase beta. The latter enzyme incorporated about 1.6 molecules of dTMP (corresponding to 6 molecules od dNMP) per one correndonuclease incision. The length of the DNA polymerase beta product was comparable to the anticipated length of the DNA region within which the hydrogen bonds were disrupted owing to dimer formation. The enzyme required Mg(2)=nd four dNTPs for reaction and was resistant to N-ethylmaleimide or p-mercuribenzoate. The average numbers of deoxynucleotides incorporated per one DNAase I incision or per one nonspecific break, measured in control samples, were equal, amounting to 0.3 dTMP molecule. This value corresponded to 1.2 dNMP molecule; in our opinion, this reflects contaminating nuclease activity of the system used. The present results testify to the ability of DNA polymerase beta to repair synthesis by the "patch and cut' mechanism.