RecA protein filaments can juxtapose DNA ends: an activity that may reflect a function in DNA repair

To further characterize the role of RecA protein-DNA filaments in general recombination and DNA repair, we have examined interactions of these filaments with themselves following formation. When linear double-stranded DNA was incubated with RecA in the presence of Mg2+ and adenosine 5'-[gamma-thio]triphosphate, monomer-length (1n) nucleoprotein filaments were observed. Following continued incubation, filaments having 2n, 3n, ... lengths were observed, indicating that an end-to-end joining of the monomer-length filaments had occurred. When linear single-stranded DNA was covered by RecA protein under several conditions, the ends of the resulting filaments joined together rapidly, producing circular filaments. The end-to-end joining of single-stranded DNA-RecA filaments appeared to require that 3' DNA ends be juxtaposed with 5' DNA ends, because double-stranded DNA molecules having long single-stranded DNA tails with only 3' or 5' termini did not join end-to-end. However, when both 5' and 3' ends were present in the reaction, joining was observed. We suggest that this end-to-end joining activity may help explain the role of RecA protein in both the protection of damaged DNA ends and the repair of double-stranded DNA breaks.