Unidirectional branch migration promoted by nucleoprotein filaments of RecA protein and DNA

Described above are the initial results from a series of experiments designed to more carefully define the mechanism of RecA protein-promoted DNA strand exchange, and in particular, the branch migration phase of this reaction. We have also presented two general models for RecA protein-promoted branch migration. Although the data required to demonstrate a particular mechanism are presently unavailable, we believe these models will provide a useful framework to suggest future experiments. The models also suggest possible explanations for observations that are presently unexplained. In particular, it is possible to explain the observed inefficiency of ATP hydrolysis in this system without invoking the notion that much of the ATP hydrolysis observed is simply uncoupled. If the utilization of extensive filaments of RecA protein to carry out branch migration is inherently inefficient, why might such a mechanism be employed by the cell? The answer to this question may lie in the role of RecA protein-promoted branch migration in post-replication repair (West et al. 1981c). It might be expected that the branch points in a RecA protein-promoted branch migration reaction are susceptible to nuclease degradation. Such degradation would halt branch migration and prevent postreplication repair. One of the roles of the RecA nucleoprotein filament may be to protect the branch point from nuclease degradation and ensure formation of an extensive region of heteroduplex DNA. The apparent energetic cost of this reaction may be a relatively small price to pay to ensure the repair of a potentially lethal DNA lesion.