Non-essential UV-sensitive bacteriophage T4 mutants affecting early DNA synthesis: a third pathway of DNA repair

Properties of bacteriophage T4 proteins deficient in replication repair

An epistasis group of mutations engendering increased sensitivity to diverse DNA-damaging agents was described previously in bacteriophage T4. These mutations are alleles of genes 32 and 41, which, respectively, encode a single-stranded DNA-binding protein (gp32) and the replicative DNA helicase (gp41). The mechanism by which the lethality of DNA damage is mitigated is unknown but seems not to involve the direct reversal of damage, excision repair, conventional recombination repair, or translesion synthesis. Here we explore the hypothesis that the mechanism involves a switch in DNA primer extension from the cognate template to an alternative template, the just-synthesized daughter strand of the other parental strand. The activities of the mutant proteins are reduced about 2-fold (for gp32) or 4-fold (for gp41) in replication complexes catalyzing coordinated synthesis of leading and lagging strands, in binding single-stranded DNA, promoting DNA annealing, and promoting branch migration. In striking contrast, the mutant proteins are strongly impaired in promoting template switching, thus supporting the hypothesis of survival by template switching.

Bypass of pyrimidine dimers in DNA of bacteriophage T4 via induction of primer RNA

Bacteriophage T4 has a third pathway for repair of damaged DNA besides excision repair and recombination repair. This pathway is a mechanism for the toleration of lesions rather than the repair of lesions. The substrate for this process is gapped DNA copied from a damaged template. Evidence indicates that these gaps are filled, giving rise to daughter strands that are sensitive to heat and to treatments with RNAase. These daughter strands subsequently serve as templates for DNA that is resistant to RNAase. This third pathway is dependent upon gene 41 (RNA-priming protein), gene uvsZ (function unknown) and gene 30 (polynucleotide ligase) and is presumed to consist of 4 steps: (1) induction of primer RNA opposite the lesion in the template; (2) elongation of primers by DNA polymerase; (3) ligation of daughter-strand fragments, without removal of primer RNA; (4) replication of DNA carrying RNA sequences, giving homogeneous DNA strands. We have called this process 'Re-initiation repair'.

Participation of bacteriophage T4 gene 41 in replication repair

The location of the non-essential T4 mutant uvs79, with defective replication repair, is described. After crosses with double mutants dispersed over the early region of T4, a linkage was observed with the double mutant am41 : am42. For more accurate location, crosses were made with single mutants. Uvs79 proved to be located between mutants amC23 and amN81 in gene 41, as shown by 3-point crosses. No genetic complementation with respect to multiplicity reactivation was found between amN81 and uvs79 after co-infection of an su- host. Apparently, mutant amN81 is disturbed as to replication repair and, owing to its lack of DNA synthesis, also in replication-dependent recombination repair. Consequently, the product of gene 41 has a function additional to its RNA-primer induction during replication of undamaged DNA. Presumably, the product of gene 41 induces RNA primers opposite DNA regions containing lesions. This capability is believed to be specifically affected by the uvs79 mutation.