Hysteretic regulation of Ustilago rec1 protein during synapsis.

rec1 protein slowly loses activity in synaptic pairing reactions. The explanation for this result would appear to be that the overall reaction is limited by a slow transition of the protein to an inactive state. When preincubated with single-stranded DNA and ADP, rec1 protein forms a dead-end complex that is unable to promote homologous pairing. Its pairing activity can be restored by lowering the temperature for several minutes or can be maintained if duplex DNA sharing no homology with the single-stranded DNA is also included. These results suggest a mechanism for attenuating pairing activity during recombination. After recognition of homologous sequences, rec1 protein undergoes a slow, ligand-induced conformational change to an inactive form.