Primer-mediated inhibition of the hydrolysis of template DNA by T5-induced DNA polymerase

Interaction of a DNA-binding protein, the product of gene D5 of bacteriophage T5, with double-stranded DNA: effects on T5 DNA polymerase functions in vitro

The gene D5 product (gpD5) of bacteriophage T5 is a DNA-binding protein that binds preferentially to double-stranded DNA and is essential for T5 DNA replication, yet it inhibits DNA synthesis in vitro. Mechanisms of inhibition were studied by using nicked DNA and primed single-stranded DNA as a primer-template. Inhibition of T5 DNA polymerase activity by gpD5 occurred when double-stranded regions of DNA were saturated with gpD5. The 3' leads to 5' exonuclease associated with T5 DNA polymerase was not very active with nicked DNA, but inhibition of hydrolysis of substituents at 3'-hydroxyl termini by gpD5 could be observed. T5 DNA polymerase appears to be capable of binding to the 3' termini even when double-stranded regions are saturated with gpD5. The interaction of gpD5 with the polymerases at the primer terminus is apparently the primary cause of inhibition of polymerization.

Mechanism of 3' to 5' exonuclease associated with phage T5-induced DNA polymerase: processiveness and template specificity

T5-induced DNA polymerase has an associated 3' to 5' exonuclease activity. Both single-stranded and duplex DNA are hydrolyzed by this enzyme in a quasi-processive manner. This is indicated by the results of polymer-challenge experiments utilizing product analysis techniques. Due to the quasi-processive mode of hydrolysis, the kinetics of label release from the 3'-terminally labeled oligonucleotide substrates, annealed to complementary homopolymers, show an initial high rate of hydrolysis. In the case of both single-stranded and duplex DNA substrates, hydrolysis seems to continue, at best, up to the point where the enzyme is five or six nucleotides away from the 5-end. The enzyme carries out mismatch repair, as evidenced by experiments with primer molecules containing improper base residues at the 3'-OH terminus. Control experiments with complementary base residues at the 3'-end indicate that extensive removal of terminal residue takes place in the presence of dNTP's only when such residues are "improper" in the Watson-Crick sense.