While the E. coli RecA protein has been the most intensively studied e
nzyme of homologous recombination, the unusual RecA-DNA filament has s
tood alone until very recently. It now appears that this protein is pa
rt of a universal family that spans all of biology, and the filament t
hat is formed by the protein on DNA is a universal structure. With Rec
A's role in recombination given new and greatly increased significance
, we focus in this review on the energetics of the RecA-mediated stran
d exchange and the relation between the energetics and recombination s
panning heterologous inserts.