Transcription of a region downstream from lambda ori is required for replication of plasmids derived from coliphage lambda.

DNA replication of lambda phage depends on transcriptional activation at or around the lambda ori region by RNA polymerase. To elucidate the function of the transcriptional activation, we constructed several plasmids carrying lambda ori and lacP, whose relative locations and directions were different from each other, and studied replication activity of these recombinant plasmids. Transcription in a region immediately downstream from lambda ori, but not in the lambda ori region, was found to be essential for plasmid replication. Transcription proceeding over a certain minimal length was required and only rightward-directed transcription was effective for the activation.