The starting point and direction of rolling-circle replicative intermediates of coliphage lambda DNA.

Intermediates of lambda DNA replication in the second half of the latent period after phage lambda infection were isolated and investigated in the electron microscope by denaturation mapping. The isolated replicative forms (RF) are predominantly single branched circular DNA. The starting points of replication in these lariat molecules located at the same region as the first round lambda DNA replication. About 60% of the RF replicate from left to right and the other 40% replicate in the reverse direction. The free ends of the tails are located at many sites on the lambda genome. Replicating circles with a linear DNA tail longer than one unit length of lambda genome represent about 30% of the replicating molecules. These long linear tails (concatemers) produced by the rolling-circle (Gilbert and Dressler, 1968; Eisen et al., 1968; Skalka et al., 1972; Takahashi, 1974) are one of the best candidates for a precursor DNA of progeny phage.