Marker-dependent recombination in T4 bacteriophage. III. Structural prerequisites for marker discrimination.

Distance- as well as marker-dependence of genetic recombination of bacteriophage T4 was studied in crosses between rIIB mutants with known base sequences. The notion of a "basic recombination," which is the recombination within distances shorter than hybrid DNA length in the absence of mismatch repair and any marker effects, was substantiated. The basic recombination frequency per base pair can serve as an objective parameter (natural constant) of general recombination reflecting its intensity. Comparative studies of the recombination properties of rIIB mutants with various sequence changes in the mutated sites showed that the main factor determining the probability of mismatch repair in recombination heteroduplexes is the length of a continuous heterologous region. A run of A:T pairs immediately adjoining the mismatch appears to stimulate its repair. In the case of mismatches with DNA strands of unequal length, formed by frameshift mutations, the repair is asymmetric, the longer strand (bulge) being preferentially removed. A pathway for mismatch repair including sequential action of endonuclease VII (gp49)----3'----5' exonuclease (gp43)----DNA polymerase (gp43)----DNA ligase (gp30) was proposed. A possible identity of the recombinational mismatch repair mechanism to that operating to produce mutations via sequence conversion is discussed.