Journal of General Microbiology (I975), 91,369-375 Printed in Great Britain 369 Mutagenic DNA Repair in Escherichia cok Conditions for Error-free Filling of Daughter Strand Gaps By JORUNN E. EYFJORD, M. H. L. GREEN AND B. A. B R I D G E S MRC Cell Mutation Unit, University of Sussex, Falmer, Brighton BNI gQG (Received 28 April 1975; revised I July 1975) SUMMARY Two situations have been observed in which daughter strand gaps in DNA syn- thesized after exposure of excision-deficient Escherichia coli to ultraviolet light are filled but in which no mutations are formed as judged by loss of photoreversibility : (i) during the first 20 min of growth after U.V. irradiation, and (ii) when repair is allowed to occur in buffer. We suggest as an explanation that the majority of daughter strand gap-filling is error free and that mutations arise through a minor error-prone repair pathway which is inoperative under these conditions. INTRODUCTION When pyrimidine dimers induced by U.V. light in bacterial DNA pass through the replica- tion point they give rise to gaps in the newly-synthesized DNA. These gaps are filled during subsequent incubation by a process termed post-replication repair (Rupp & Howard- Flanders, I 968). In excision-deficient strains, dimers persist indefinitely and must all be dealt with in this way. Such strains are much more sensitive to U.V. irradiation than excision- proficient strains and low doses produce far more mutants (Hill, 1965), which led to the suggestion that mutations arise during post-replication repair either during recombinational exchanges or by enzymic insertion of incorrect bases opposite dimers (Bridges, Dennis & Munson, 1967; Witkin, 1967). Since the recA mutation abolishes both U.V. mutagenesis and post-replication repair (Witkin, 1969; Smith & Meun, 1970; Kondo, 1973) it seemed likely that mutations might arise as errors during recombinational daughter strand exchanges. If this were so, daughter strand gap-filling and mutation should occur concomitantly. More recently, however, evidenceIhas accumulated that mutations occur through a minor non-recombinational com- ponent of post-replication repair (Witkin & George, 1973 ; Doubleday, Bridges & Green, 1g75), so that mutation formation might well show different kinetics from normal daughter strand gap-filling. We attempted to distinguish between these alternatives in the excision-deficient strain E. coli WP~ uvrA. We measured daughter strand gap-filling by alkaline sucrose sedimenta- tion (McGrath & Williams, 1966) and induction of mutations by assaying reversion from Trp to Trp+. Loss of susceptibility of Trp+ revertants to photoreversal was taken to indicate the occurrence of a stage in the mutational process which was irrevocable and beyond which a mutation would form whether or not the dimer was still in the DNA.