Processivity of T4 endonuclease V is sensitive to sodium chloride concentration

Abstract

We previously reported that endonuclease V of bacteriophage T4 reacts processively with pyrimidine dimers in UV-irradiated DNA, tending to react with all of the dimers on one DNA molecule before reacting with dimers on another DNA molecule [Lloyd, R.S., Hanawalt, P.C., and Dodson, M. L. (1980) Nucleic Acids Res. 8, 5113-5127]. In this paper we show that this processivity depends upon salt concentration: it can be detected in 10 mM NaCl but not, by our methods, in 100 mM NaCl. In addition, we show that endonuclease V binds to unirradiated DNA in 10 mM NaCl but not in 100 mM NaCl. We conclude that T4 endonuclease V binds to pyrimidine dimers in a two-step process in 10 mM NaCl. It first binds electrostatically to underdamaged sections of DNA, and it remains bound during the second step in which it "searches" for pyrimidine dimers. Our conclusion is analogous to the expanded target theory developed for Lac repressor [Berg, O.G., Winter, R.B., and von Hippel, P.H. (1981) Biochemistry 20, 6929-6948].