Mechanisms O F Spontaneous and Induced Frameshift Mutation in Bacteriophage

Frequencies of spontaneous and proflavine-induced frameshift mutations increase dramatically as a function of the number of reiterated base pairs at each of two sites in the lysozyme gene of bacteriophage T4. At each site, proflavine induces addition mutations more frequently than deletion mutations. We confirm that the steroidal diamine, irehdiamine A, induces frameshift addition mutations. At sites of reiterated bases, we propose that base pairing is misaligned adjacent to a gap. The misaligned configuration is stabilized by the stacking of mutagen molecules around the extrahelical base, forming a sandwich. Proflavine induces addition mutations efficiently at a site without any reiterated bases. Mutagenesis at such sites may be due to mutagen-induced stuttering of the replication complex. UTATIONS in DNA that result in the addition or deletion of one or M more bases (BRENNER et al. 1961) are ubiquitous. In bacteriophage T4 they represent the most common spontaneous class, and they have been identified in bacteria (AMES and WHITFIELD 1966), yeast (STEWART and SHERMAN 1974) and humans (FITCH 1973). Because the addition or deletion of bases (except in multiples of three) results in a shift in the reading frame for the translation of the genetic message (CRICK et al. 196 l) , such mutations are often referred to as frameshifts. Frameshift mutations occur with especially high frequencies in regions of repeated base sequences (OKADA et al. 1972; PRIBNOW et al. 1981; FARABAUCH et al. 1978). Previously (STREISINGER et al. 1966), we suggested a model that accounts for this observation; Figure 1 shows the steps we propose. Since frameshift mutations occur near ends of DNA molecules in T4 (DRAKE 1964; STRICINI 1965; LINDSTROM and DRAKE 1970), we indicate the creation of an end (a) and the subsequent digestion of one strand of the DNA molecule (c). If digestion stops adjacent to a stretch of reiterated bases, the melting of this stretch (d) could be followed by reannealing out of register, leaving one or more extrahelical bases bulged out of the duplex (e, f ) . When the undigested strand of DNA is used as template and the end of the complementary strand is used as primer, DNA synthesis restores a double-stranded molecule that is ' Deceased August 1 1 , 1984. Send reprint requests to: Institute of Molecular Biology, University of Oregon, * Present addi-rss: 2830 Kinerald Sti-rec, Eugene, Oregon 97403. Eugene, Oregon 97403. Genetics 109 633-659 April, 1985 634 G. STREISINGER AND J. (E.) OWEN