Effects of p-fluorophenylalanine on the induction of mutations in bacteriophage T4. II. Nitrous acid mutagenesis

The modification of induced genetic change in yeast by an amino acid analogue

Treatment of diploid yeast cultures with the amino acid analogue, para-fluorophenylalanine (PFPA), at concentrations which caused inhibition of growth, resulted in up to 5 fold increases in the frequency of mitotic gene conversion at two different heteroallelic loci. With haploid yeast cultures, growth in PFPA increased the rate of forward mutation to canavanine resistance by at least 2 fold. Growth of diploids in PFPA prior to exposure to the deaminating agent nitrous acid, the cross-linking agent mitomycin C, the alkylating chemical ethylmethanesulphonate (EMS) and UV light resulted in significant changes in the potency of these diverse mutagens to induce intragenic recombination. For all four mutagens, increased frequencies of gene convertants/viable cell were observed in those cultures which had been exposed to the amino acid analogue prior to mutagen treatment. In haploid WT yeast cells, amino acid analogue incorporation resulted in an enhanced frequency of UV induced forward mutation to canavanine resistance whilst in a DNA repair deficient rad 6 mutant this interaction between UV and PFPA was abolished. The results have been interpreted on the basis of incorporation of the analogue into enzymes involved with DNA replication with a consequent loss of fidelity of such enzymes and increased errors in base incorporation.

The dependence of HNO2 mutagenesis in phage T4 on ligase and the lack of dependence of 2AP mutagenesis on repair functions

A temperature sensitive ligase allele of phage T4 reduced or eliminated HNO2 induced reversion of am mutants. Since at the temperatures used, the ligase mutant is defective in the repair of some types of lethal lesions (i.e., UV, MMS and EMS induced lesions) these results indicate that HNO2 mutagenesis may occur through a ligase dependent repair pathway. In contrast, 2AP induced mutation was not inhibited by mutants defective in the gene 30 ligase or in genes 32, 39, 41, 42, 44, 45, 46, 47, 49, 52, 56, 58-61 and v. This indicates that 2AP mutagenesis probably does not depend on a repair pathway in phage T4.

Intragenic mutational spectra and hot spots

In this review we outline the various factors which may contribute to the non-randomness of intragenic mutational spectra and the occurrence of hot spots. These factors include sample size limitation, particularly for sites of low mutability, and possible regions of low recombination potential. In addition, the nature of the gene product places great restraint on the detectability of either frameshift and premature chain-terminating mutations on one hand, or of the majority of missense mutations on the other. The nature of the Genetic Code itself also limits the mutational spectrum in so far as specific base pair substitutions lead only to a limited number of detectable amino acid replacements. Mutational hot spots may be a special example of the influence of neighbouring base pairs in the mutability of any given base pair. This is apparently true for frameshift mutations which tend to occur in runs of repeated base pairs or base pair doublets. Neighbouring base effects could operate not only at the level of initial reactivity with a mutagen, but also subsequently at the levels of DNA repair, recombination or replication. In some cases rare or modified bases may be responsible for neighbour effects. We suggest specific experimental approaches which seem likely to aid in the elucidation of these problems.

Effects of p-fluorophenylalanine on the induction of mutations in bacteriophage T4. I. 5-Bromouracil mutagenesis

The amino acid analogue rho-fluorophenylalanine (PFPA) was found to have no mutagenic activity in the gamma system of bacteriophage T4. However, under standard conditions for 5-bromouracil (5-BU) mutagenesis, PFPA depressed the induced frequencies for both forward and reverse mutations. When the folate antagonist sulphanilamide (SU) was omitted from the mutangenic treatment medium or when it was replaced by Trimethoprim (TM), another folate antagonist, this depressive effect was abolished. It was proposed that PFPA alleviated the inhibitory action of SU.