Daughter origins can be held together by O protein in phage lambda replicative intermediates.

When lambda replicative intermediates are incubated with initiator protein O, complex molecules are observed in which O interacts with both daughter origin segments and with growing points. In the simplest of these molecules it appears that daughter origins and growing points may all be bound together at a single point. When replicative intermediates are sequentially incubated with single-stranded binding protein and O protein, simpler structures are observed. In this case, both daughter origins are bound together by O protein. This result mimics that found when plasmid containing two tandem lambda origin sequences is reacted with O protein. In this case double origin binding produces a DNA loop. Double origin binding, as demonstrated in this investigation, creates the potential for topological domains which will have important effects on the ability of daughter origins to initiate replication.