An analysis of replication errors made by a defective T4 DNA polymerase

Summary

An amber mutant in the head protein of bacteriophage T4D, amH36 has been induced to revert by a mutator, tsL56 in gene 43 (the structural gene for DNA polymerase, de Waard, Paul and Lehman, 1965) which is known to cause errors in replication. As a consequence the known am base triplet is converted to other triplets which assign certain amino acids. The nature of the replication errors has been analyzed by looking at the insertion of amino acids in a peptide from the head protein of 60 independent am ⁺ revertants. Of these, 38 had incorporated tyrosine (like spontaneous revertants also do) while in 21 cases glutamine was inserted and in one case glutamic acid. With the help of the codon catalogue it could be shown that the L56 polymerase promotes an A:T→G:C transition as well as more than one type of transversion. The single revertant which had incorporated glutamic acid clearly represents an A:T→C:G transversion. The other transversions leading to the insertion of tyrosine indicate that a C:G pair has been converted. In this case the degeneracy of the code does not allow to differentiate between the transversion C:G→G:C and C:G→A:T. These findings and the absence of certain amino acids as permissible substituents are discussed with regard to the specificity of the errors in replication made by L56 polymerase.