Sequence-specific pausing during in vitro DNA replication on double-stranded DNA templates.

Sequence-specific pausing occurs during DNA synthesis catalyzed by the bacteriophage T4 DNA polymerase holoenzyme in the presence of the T4 helix destabilizing protein (gene 32 protein). Two of the six strongest pause sites on a double-stranded bacteriophage fd DNA template are in regions where hairpin helices are predicted to form when the DNA is single stranded. However, the other pause sites are in regions that are not obviously involved in secondary structure. The positions of the DNA chain ends produced at one pause site of each type were determined to within +/- 2 nucleotides. At this resolution, a clustering of sites is observed, suggesting that the polymerase holoenzyme may become destabilized when moving along selected regions of the DNA and then pause at one or more of several closely spaced positions. The addition of the T4 gene 41 protein (a DNA helicase that forms part of the T4 primosome) to the above replication system greatly increases the rate of fork movement and eliminates detectable pausing. In contrast, the addition of the T4 dda protein (a second DNA helicase that increases the rate of fork movement to a similar extent) has no affect on replication fork pausing. This difference could either be due to specific protein-protein interactions formed between the polymerase holoenzyme and the 41 protein or to the highly processive movement of the 41 protein along the displaced DNA strand.