On the mechanism of ssDNA recombination

The current model for ssDNA recombination is that upon entering the cell, the recombining oligo is coated with Beta protein, protecting it from degradation. The Beta-coated oligo then finds and anneals to complementary ssDNA.

Figure 3. Model for ssDNA recombination. I.) As an oligo enters the cell, it is coated by Beta protein which protects it and mediates annealing with complementary ssDNA. In this diagram, the target for annealing is the DNA replication fork. DNA replication is occurring from left to right and the newly synthesized DNA on the lagging-strand is denoted by the short fragments. II.) The Beta-coated recombining oligo anneals and this oligo is joined to the lagging-strand at the DNA replication fork. If the changes within this oligo escape MMR, then after another round of replication the mutation is permanent.

Several lines of evidence indicate that this ssDNA target is present at the DNA replication fork. First, DNA replication is required for ssDNA recombination. Second, of the two complementary oligos that can alter any sequence, the lagging-strand oligo is always more efficient. During DNA synthesis, the lagging-strand has more exposed ssDNA. Third, the MMR system has a large effect on ssDNA recombination, and this system functions at or near the DNA replication fork soon after the fork has passed a given sequence. By this model, after the oligo anneals at the DNA replication fork, DNA ligase joins the oligo to the surrounding Okazaki fragments. As expected, DNA ligase mutations reduce ssDNA recombination.