We plan to investigate the mechanisms of postreplication and recombinational repair in irradiated bacteria and in in vitro systems containing extracts of the cells or purified enzymes. We will focus on the interaction between damaged DNA molecules and intact covalent circular homologous DNA. Molecules carrying incised crosslinks, or other single strand gaps, appear to initiate genetic exchanges with the undamaged homologs at a greatly enhanced frequency, causing many of them to be cut and converted to structures other than covalent circles. We plan to purify recA protein from E. coli carrying recA ion on a plasmid, and to investigate the role of this protein in the cutting reaction just described. It appears that recA protein is not an endonuclease, but needs other cell constituents for the cuttin in trans reaction. Our investigations will include studies of the binding of recA protein to DNA molecules of various specific structures, such as incised crosslinked molecules, the ATPase activities, and the sequence of events as it promotes specific pairing between the gapped molecules and undamaged homologs.