A unified mechanism for the nuclease and unwinding activities of the recBC enzyme of Escherichia coli.

Using a gentle method to prepare complexes of duplex DNA and the recBC enzyme for electron microscopy, structures not seen previously were observed to be associated with the double strand DNA exonuclease activity of the enzyme. These were terminal forms and loop + tail(s) structures. Both individual terminal single-stranded tails and single-stranded regions within duplexes were also observed. Observation of the terminal single-stranded structures present after various reaction conditions and after various potentially disruptive treatments helped to identify those structures which might represent true reaction intermediates. Based on these results and those of previous studies, a modified model for the mechanism of recBC enzyme action on double-stranded, linear DNA is proposed.