The role of gene O protein in the replication of bacteriophage lambda

The role of the product of gene O of bacteriophage lambda in phage DNA replication was examined by shifting cells infected with an Ots mutant to the nonpermissive temperature after incubation at the permissive temperature. Thymidine incorporation after the temperature shift exhibits biphasic kinetics, with rapid synthesis immediately after the shift and slower synthesis 2-15 min after the shift. Following a shift to the nonpermissive temperature early in infection, the proportion of replicative intermediates decreases substantially and sigma-structures are favored for preservation. When the shift is done late in infection, the proportion of replicative intermediates remains the same. The average length of single-stranded regions at the branch points increases after a shift to the nonpermissive temperature. Most of the counts which are incorporated after the temperature shift are incorporated into strands which are longer than unit length. These results favor a model in which lambda O protein is required for the initiation of replication, but at least some elongation can continue in the absence of O. It is possible that O protein plays a role in elongation of the lagging strand at replicative forks. This model suggests a way to regulate the transition between theta and sigma replication which occurs as lambda infection proceeds.