PROCESSING OF INTERMEDIATES IN RECOMBINATION AND DNA-REPAIR - IDENTIFICATION OF A NEW ENDONUCLEASE THAT SPECIFICALLY CLEAVES HOLLIDAY JUNCTIONS

The formation and subsequent resolution of Holliday junctions are crit ical stages in recombination. We describe a new Escherichia coli endon uclease that resolves Holliday intermediates by junction cleavage. The 14 kDa Bus protein binds DNA containing a synthetic four-way junction (X-DNA) and introduces symmetrical cuts in two strands to give nicked duplex products. Rus also processes Holliday intermediates made by Re cA into products that are characteristic of junction resolution. The c leavage activity on X-DNA is remarkably similar to that of RuvC. Both proteins preferentially cut the same two strands at the same location. Increased expression of Bus suppresses the DNA repair and recombinati on defects of ruvA, ruvB and mvC mutants. We conclude that all ruv str ains are defective injunction cleavage, and discuss pathways for Holli day junction resolution by RuvAB, RuvC, RecG and Rus.