Printed m U.S.A. ADP-mediated Dissociation of Stable Complexes of recA Protein and Single-stranded DNA*

The complete exchange of strands between circular single-stranded and full length linear duplex DNAs promoted by the recA protein of Escherichia coli is dependent upon the hydrolysis of ATP and is strongly stimulated by the single-stranded DNA binding protein (SSB). In the presence of SSB, stable complexes of recA protein and single-stranded DNA are formed as an early step in the reaction. These complexes dissociate when the AI)P/ATP ratio approaches a value of 0.6-1.5, depending upon reaction conditions. Thus, ATP hydrolysis never proceeds to completion but stops when 40-60 % of the input ATP has undergone hydrol-ysis. recA protein can participate in a second round of strand exchange upon regeneration of the ATP. While 100-200 mol of ATP are hydrolyzed/mol of heterodu