Heteroduplex formation and polarity during strand transfer promoted by Ustilago rec 1 protein.

Ustilago rec1 protein promotes formation of joint molecules between circular strands and linear duplex DNA in two stages. During the first stage--synapsis--ATP, or the nonhydrolyzable analog adenylyl-imidodiphosphate, is a necessary cofactor in the formation of complexes. During the second stage--strand exchange--transfer of the circular single strand to the duplex appears to require energy derived from ATP hydrolysis. Rec1-promoted strand exchange is driven in a unique direction. Circular plus strand pairs with the 5' end of its complement in the linear duplex molecule. The 3' end of the plus strand is displaced from the duplex as the circle rolls in.