Reactions between a single-stranded DNA (ssDNA) and a double-stranded
DNA (dsDNA) provide an efficient model to study RecA promoted homologo
us recombination. We have devised an assay in which the ssDNA is first
bound to a nitrocellulose membrane. RecA protein is loaded on this me
mbrane (loading step) which is then incubated with a labelled homologo
us dsDNA (incubation step). Since this assay can be used for study of
mutant RecA proteins or RecA-like activities in crude extracts from ot
her organisms, we have characterized the reaction promoted on the memb
rane. Under these new conditions, the reaction keeps the main characte
ristics observed with classical assays performed in solution: increasi
ng NaCl concentration destabilized the RecA-DNA complex, ATP gamma S w
as required for formation of stable RecA-DNA complex, initiation of th
e reaction exhibits the same polarity as in classical assays, a comple
te strand exchange with a 44 bp long duplex oligonucleotide has been r
ecorded under our conditions. Moreover, our results indicate that the
binding of RecA protein itself to the nitrocellulose membrane did not
impair its ability to promote homologous pairing. Pairing reactions in
volving long dsDNA (6407 bp) were more efficient with hydrolysable ATP
than with ATP gamma S only when the ssDNA was bound to the membrane.
Furthermore, ATP hydrolysis was not required when using short dsDNA(44
bp). These results constitute experimental support for a new role for
the ATPase activity of RecA protein: the energy produced could favor
the initiation of RecA mediated recombination involving long stretches
of DNA which have restricted freedom to rotation.