The pairing activity of stable nucleoprotein filaments made from recA protein, single-stranded DNA, and adenosine 5'-(gamma-thio)triphosphate.

Under conditions that diminish secondary structure in single-stranded DNA, stable presynaptic filaments can be formed by recA protein in the presence of the nonhydrolyzable analog ATP gamma S, without the need for Escherichia coli single strand binding protein. Such stable presynaptic filaments resemble those formed in the presence of ATP and pair efficiently with homologous duplex DNA. Since this kind of stable filament does not displace a strand from the duplex molecule, it provides a model substrate to study synapsis independent of the earlier and later stages of the recA reaction. Even though detectable strand displacement did not occur in the presence of ATP gamma S, both single strand and double strand breaks in duplex DNA stimulated homologous pairing. These and related observations support the view that the presynaptic nucleoprotein filament and naked duplex DNA intertwine to form a nascent joint in which the duplex DNA is partially unwound, i.e. in which the pitch of the involved duplex segment is reduced.