Lambdap(o), a promoter for oop RNA synthesis, has a role in replication of plasmids derived from bacteriophage lambda

Transcription initiated at the bacteriophage lambdap(o) promoter gives a short RNA, called oop RNA. Early studies led to a proposal that this transcript plays a role in the initiation of lambda DNA replication. In fact, the p(o) promoter is located in the lambda replication region and it was suggested that oop RNA may be a primer for replication proceeding leftward from orilambda. However, since in vitro experiments demonstrated that primers for lambda DNA replication are produced by the dnaG gene product (DnaG primase) and subsequent in vivo studies indicated that oop RNA is an antisense RNA for the lambda cII gene expression, the above-mentioned hypothesis has fallen into oblivion. Nevertheless, here we demonstrate that the p(o) promoter plays a role in lambda DNA replication, indeed. We found that lambda plasmids bearing a mutation that inactivates p(o) occur in Escherichia coli cells in a copy number significantly lower than wild-type lambda plasmids. Amplification of lambdap(o)(-) plasmids during the relaxed response was less efficient relative to lambdap(o)(+) plasmids suggesting less frequent initiation of replication from orilambda in the absence of transcription from p(o). This suggestion was confirmed by measurement of incorporation of [(3)H]thymidine into lambda plasmid DNA during pulse-labeling experiments. Therefore, we propose that transcription from the p(o) promoter stimulates replication initiation at orilambda as suggested a long time ago, however, contrary to that suggestion, we assume that the process of p(o)-initiated transcription per se but not the transcription product (oop RNA) might play a role at early steps of lambda DNA replication.