INDUCTION OF SINGLE‐STRAND BREAKS (ALKALI‐LABILE BONDS) IN BACTERIAL AND PHAGE DNA BY NEAR UV (365 nm) RADIATION*

Abstract —Irradiation at 365 nm results in the induction of approximately 2–4 times 10 ‐6 and 1‐2times 10 ‐6 single‐strand breaks (alkali‐labile bonds) per 10 8 daltons per J m ‐2 in extracted phage T4 DNA and in Escherichia coli bacterial DNA, respectively. The rate of break induction in DNA of intact phage is approximately one‐fourth that for extracted phage DNA. 2‐aminoethylisothiouronium bromide‐HBr protects against break induction in both phage systems. No breaks are induced in the DNA of bacteria irradiated under anaerobic conditions over the dose range tested. Possible induction mechanisms are suggested. Consideration is given to the relative importance of pyrimidine dimers and single‐strand breaks in the bactericidal action of 365 nm radiation.