PROPERTIES OF RECA COMPLEXES WITH HOMOPOLYMERIC DNA STRANDS DEPEND ONSEQUENCE COMPLEMENTARITY - IMPLICATIONS FOR THE MECHANISM OF STRAND EXCHANGE

We have characterised complexes between RecA and single-stranded homop olynucleotides by linear dichroism spectroscopy and small angle neutro n scattering to investigate base pairing possibilities among DNA stran ds bound in a RecA filament. We find that in the presence of the non-h ydrolysable cofactor ATP gamma S, and very likely also in the presence of ATP, a RecA fiber has three distinct DNA binding sites, each of wh ich can bind one strand of DNA at a stoichiometry of three nucleotides per RecA monomer. The structural and hydrodynamic properties of the c omplexes are found to depend on the number of strands bound and on seq uence complementarity among the strands. For example, RecA-homopolyme r(3)-ATP gamma S complexes aggregate when either of the strands bound in sites I and II is complementary to the strand bound in site III. W e have also studied the RecA catalysed annealing of complementary homo polymers and find it to be most efficient when two strands of one homo polymer are bound per RecA filament prior to the addition of the compl ementary homopolymer. These results suggest that a DNA strand bound in site III can base-pair with either of the strands in sites I and II, whereas the latter strands are unable to base-pair with each other.