Polymerase Infidelity and Frameshift Mutation

Summary

Mutant T4 DNA polymerases which alter mutation rates in vivo have been used to approach questions of replication fidelity. Most studies have characterized “mutator” or “antimutator” polymerases by their influence upon base-pair substitution mutation, particularly transitions. We are extending the characterization of mutant polymerases to include the role of T4 DNA polymerase in frame fidelity.

We conclude that T4 DNA polymerase indeed plays a major role in frame fidelity from the observat ion that among 26 ts DNA polymerase alleles more than half increased or decreased the revertant frequency of an rll frameshift between 3- and 400-fold. Furthermore, the revertant frequency at this frameshift site was not altered by some polymerases known to increase base-pair substitution frequencies. Thus, polymerase mutants may provide a means to separate fidelity elements which are uniquely important to either frameshift or to base-pair substitution mutations.

Our approach provides a detailed description (including frequency, location, and addition or deletion character) of frameshifts produced in the presence of different polymerases. The frameshifts arise in the T4 rllB gene and are recognized as “revertants” or “suppressors” of other T4 rllB frameshifts having a defined genetic sign (+ or −). From several initial tester frameshifts, spectra of suppressor frameshifts have been determined for a number of polymerases. The frameshift spectra of mutant polymerases are distinct from the spectrum of the wild-type polymerase. Furthermore, the mutant polymerases have individually distinct propensities for pro-ducing frameshifts at particular genetic loci. This specificity has allowed us to correctly predict the influence of several mutant polymerases on frameshifts other than those originally tested and suggests an important correlation with particular fidelity defects and frameshifts at particular sites. We believe it particularly noteworthy that some polymerase mutants exhibit a distinct bias in their influence upon the frequency of frameshifts having different genetic signs. We plan to sequence these frameshifts to determine their addition or deletion nature and thereby gain a more precise molecular view of the frame fidelity defects of these mutant polymerases.