Monomers of purified RecA protein polymerize into helical fibers whose pitch is 7.2
nm to 7.5 nm and whose diameter is 11 nm. Either short (approximately 0.2 micron),
single fibers, or bundles of aligned, longer fibers, can be formed preferentially,
by varying the Mg2+ concentration. When RecA protein is bound to circular, single-stranded
phi X174 DNA it forms helical fibers of different classes of contour lengths, ranging
from 0.98 micron, depending upon the conditions of assembly. Two different helical
pitches are found, one of 9.3 nm when the incubation buffer contains, besides the
obligatory Mg2+, either ATP gamma S or ATP accompanied by single-strand binding protein,
and one of 5.5 nm when the latter additives are omitted. Preformed fibers of the compact
type can be converted to open ones of 9.3 nm pitch upon addition of ATP gamma S, even
after the removal of unbound RecA. All signs of helicity are obliterated upon glutaraldehyde
cross-linking except in those fibers whose assembly has been mediated by ATP gamma
S. RecA protein and single-strand binding protein are competitively bound to single-stranded
DNA. Composite complexes, however, are not encountered unless ATP gamma S is present.
Otherwise, segments of DNA that are coated by one or the other protein are seen as
separate regions. When the assembly of complexes of single-stranded DNA and RecA is
mediated by single-strand binding protein and ATP, the axial separation between successive
bases is 0 X 42 nm, somewhat greater than the axial distance between bases in one
strand of duplex DNA in the B form. It is proposed that the bases of the single-stranded
DNA in the complex are located near its inner surface, and that base-pairing with
double-stranded DNA takes place following invasion of the central cavity of the complex.