Left-handed DNA and the synaptic pairing reaction promoted by Ustilago rec1 protein.

Left-handed Z-DNA binds tightly to Ustilago rec1 protein. The binding reaction is strongly dependent on ATP, but complexes formed are rapidly dissociated by ADP. The parallel between the kinetics of Z-DNA binding and the synaptic pairing reaction leading to paranemic joint molecules suggests that formation of nascent heteroduplex structures in recombination is coupled with formation of left-handed Z-like DNA on the protein. Equilibrium and kinetic studies show that rec1 protein appears to have a strong Z-DNA binding site that binds Z-DNA 75 times tighter than the B form of the DNA. We propose that DNA with a structure approximated best by a left-handed Z-DNA conformation is a key intermediate in homologous pairing promoted by rec1 protein.