Transcriptional activation of ColE1 DNA synthesis by displacement of the nontranscribed strand.

Plasmid ColE1 can replicate using RNAase H and DNA polymerase I. However, it can also replicate in the absence of these enzymes. In this case, formation of a persistent hybrid between a transcript (RNA II) and the DNA indirectly activates subsequent DNA synthesis, instead of providing a primer as it does in the presence of these enzymes. To activate DNA synthesis, a certain length is required for the hybridized region and the region of minimum length cannot include a palindrome. These results show that the single-stranded region of DNA displaced by the hybridization is responsible for the activation. A single-stranded region was identified on the nontranscribed strand by its enhanced reactivity to dimethyl sulfate. The necessary length for the single-stranded region is at least 40 nucleotides. The region probably provides a site for initial binding of a helicase that further unwinds the template DNA for initiation of DNA synthesis.