Frameshift mutation in the lysozyme gene of bacteriophage T4: Demonstration of the insertion of four bases and the preferential occurrence of base addition in acridine mutagenesis

Three mutant strains of bacteriophage T4, each of which carried two frameshift mutations were isolated and the amino acid sequences of their lysozyme proteins were analyzed. The changes of the primary structure were found to be from Ala 74-Val- Arg 76 to Gly-Cys-Cys-Cys in eJ28eJD2, from Ala 73-Ala-Val-Arg- Gly 77 to Gly-Cys-Cys-(Cys, Gly) in eJD6eJD2 and from Ala 73 to Val-Asp in eJD8eJ28. From the codon assignments obtained from in vitro studies, these alterations in the amino acid sequences may be consistently explained by the following mutational events: (+ 4, -1) in eJ28eJD2, (+1, -1) in eJD6eJD2 and (-1, + 4) eJD8eJ28. We have so far analyzed 15 pseudo-wild type strains of phage T4 which carried two or three frameshift mutations in the lysozyme locus and have demonstrated the addition and the deletion of one or two bases. In this communication we report that the acridine-type mutagen, proflavin, induces the addition of not only one and two bases but also of four bases. When we summarize the base changes due to frameshift mutations in T4 phage lysozyme and tryptophan synthetase of Escherichia coli, we find that acridines predominantly cause base additions and that spontaneous mutations induce additions and deletions of bases with almost equal frequency. The present analysis further indicates that the following codons are utilized by T4-infected E. coli in vivo: Ala, GCU; Val, GUU; Arg, CGU or CGC in the wild-type and Asp, GAU; Gly, GGC and GGA; Ala, GCU; Cys, UGU and UGC; Val, GUG in the mutant strains.