A CHIMERIC REC-A PROTEIN THAT IMPLICATES NON-WATSON-CRICK INTERACTIONS IN HOMOLOGOUS PAIRING

The helical filament formed by RecA protein on single-stranded DNA pla ys an important role in homologous recombination and pairs with a comp lementary single strand or homologous duplex DNA. The RecA nucleoprote in filament also recognizes an identical single strand. The chimeric p rotein, RecAc38, forms a nucleoprotein filament that recognizes a comp lementary strand but is defective in recognition of duplex DNA, and is associated with phenotypic defects in repair and recombination. As de scribed here, RecAc38 nucleoprotein filament is also defective in reco gnition of an identical strand, either when the filament has within it a single strand or duplex DNA. A model that postulates three DNA bind ing sites rationalizes these observations and suggests that the third binding site mediates non-Watson-Crick interactions that are instrumen tal in recognition of homology in duplex DNA.