Photoreactivation of bacteriophage phi chi 174.

U.V.-irradiated single-stranded DNA-bacteriophage phi chi 174 shows a photoreactivable sector of 0-17. That this sector is relatively small compared with those for the double-stranded DNA-phages T2 and T6 (about 0-5) does not necessarily seem to be due to the pyrimidine dimers in single-stranded DNA being intrinsically a poorer substrate for the photoreactivating enzyme. This follows from the observation that intracellularly irradiated single-stranded phi chi 174 DNA also shows a photoreactivable sector of 0-4 to 0-5. The same value is obtained for intracellularly irradiated double-stranded phi chi 174 DNA. Photoreactivation of intracellularly irradiated single-stranded phi chi 174 DNA is not constant with U.V. dose for the lower dose ranged. Possible explanations for this are discussed.