Control of gene expression in the temperate coliphage 186. IX. B is the sole phage function needed to activate transcription of the phage late genes.

Using plasmid clones we have determined that the late control function B is the only phage function that is needed to activate a late promoter of coliphage 186, and we predict that it functions as an auxiliary factor to RNA polymerase in the activation of late transcription. We have also shown that a high concentration of B will activate late transcription from a prophage, and we conclude that replicating DNA is not a template requirement for B to function. The original demonstration of a need for the replication gene A in late transcription can be explained by the fact that replication leads to an increase in B gene dosage, with the consequent increase in B concentration leading to the efficient activation of the late promoters.