On the mechanism of bromouracil-induced mutagenesis

Bromouracil-induced mutagenesis in a mismatch-repair-deficient strain of Haemophilus influenzae

Cells of wild-type Haemophilus influenzae and of a mismatch-repair-deficient mutant (hex-) were grown in a chemically defined medium containing either thymidine or 5-bromodeoxyuridine (BUdR). Spontaneous mutation frequencies to resistance against 3 antibiotics observed for the thymidine cultures were 10-30 times higher for the hex- mutant. The mutation frequencies observed for the BUdR hex- culture were increased by another 10 times while those for the wild-type suspension did not differ from the frequencies seen in the thymidine medium.

Relationship between repair processes and mutation induction in bacteria

A summary is given of the main repair and replication-associated processes that can influence the induction of mutations by various mutagens in bacteria. These include both constitutive and induced, error-free and error-prone systems. The mutation yield from a treatment with a mutagen can be markedly affected by which of these systems is operating in a given bacterial species or strain. The effect of these systems on mutation induction by ultraviolet light, monofunctional alkylating agents, base analogues, and frameshift mutagens is discussed in some detail. The bearing of these studies on the practical problems of estimating hazards is briefly considered.

Genetic evidence for the nature, and excision repair, of DNA lesions resulting from incorporation of 5-bromouracil

Escherichia coli mutants defective in DNA uracil N-glycosidase (ung-) or endonuclease VI active against apurinic/apyrimidinic sites in DNA (xthA-) exhibit enhanced sensitivity towards 5-bromodeoxyuridine relative to the wild type strain, pointing to involvement of these enzymes in repair of bromouracil-induced lesions in DNA. Mutants defective in DNA polymerase I, either in polymerizing activity (polAl-) or (5' leads to 3')-exonuclease activity (polA107-) exhibit unusually high sensitivity (including marked lethality) in the presence of 5-bromodeoxyuridine. The results indicate that DNA polymerase I, and its associated (5'--3')-exonuclease activity, are involved in repair of bromouracil-induced lesions and are not readily replaced, if at all, by DNA polymerases II and III. Thermosensitive mutant in DNA ligase gene (lig ts7) shows high sensitivity towards 5-bromodeoxyuridine at 42 degrees C indicating the role of the enzyme in repair of bromouracil-induced lesions in DNA. Involvement of DNA uracil N-glycosidase, and endonuclease active against apurinic/apyrimidinic sites in recognition and repair of 5-bromouracil-induced damage permits of some inferences regarding the nature of this damage (lesions), in particular dehalogenation of incorporated bromouracil to uracil residues.

The relation of repair phenomena to mutation induction in bacteria

The relation of various processes to mutation induction by radiation and chemicals is discussed for for various species of bacteria. A variety of repair processes have been identified at the molecular level that can eliminate many kinds of potentially mutagenic lesions before they can be converted to final mutation. Fixation often but not always occurs at replication. A number of mutagens, including UV light, ionizing radiation, and a number of chemicals, induce an error-prone process, perhaps a modification of the proof-reading system, that allows bacteria to survive after potentially lethal damage at the expense of making errors. Some mutagens, notably monofunctional alkylating agents and base analogues, produce mutations by other processes. Even in these cases, repair processes play an important role. There is some evidence that error-free as well as error-prone repair processes can be induced. A brief discussion is given of the relation of these findings to the practical problems of hazards estimations.

The efficiency and extent of mutagenic activity of some new mutagens of base-analogue type

N4-Hydroxycytidine, 5-methyl-N4-hydroxydeoxycytidine and 2-amino-N6-hydroxyadenine were tested for their mutagenic activity in S. typhimurium and E. coli cells. Reversion analysis of different markers was applied in a plate-test system, and 2-aminopurine was used as a reference mutagen. (i) 2-Amino-N6-hydroxyadenine was the most potent mutagen. In some cases it gave more than 1000 colonies of revertants per plate. (ii) N6-Hydroxycytidine was the least specific mutagen. Almost all the tested markers were inducible to revert by this analogue. (iii) The mutagenic specificity of 5-methyl-N4-hydroxydeoxycytidine seemed to be opposite to that of 2-aminopurine. This suggests that the former can induce transition of CG to TA. (iv) A comparison of the mutagenic actions of N4-hydroxycytidine and 5-methyl-N4-hydroxy-deoxycytidine showed that deoxyriboside analogues are not necessarily more efficient mutagens than ribonucleosides. (v) No purine or pyrimidine deficiency was needed for mutagenesis to occur for any of the mutagens investigated. (vi) The results on bacteria with different repair abilities suggest that base-analogue mutagenesis (except perhaps for BrdUrd) occurs mainly during replication of nucleic acids containing substituted nucleosides with bi-functional specificity.

Evidence that UV-inducible error-prone repair is absent in Haemophilus influenzae Rd, with a discussion of the relation to error-prone repair of alkylating-agent damage

Haemophilus influenzae Rd and its derivatives are mutated either not at all or to only a very small extent by ultraviolet (UV) radiation, X-rays, methyl methanesulfonate, and nitrogen mustard, though they are readily mutated by such agents as N-methyl-N'-nitro-N-nitrosoguanidine, ethyl methanesulfonate, and nitrosocarbaryl. In these respects H. influenzae Rd resembles the lexA mutants of Escherichia coli that lack the SOS or reclex UV-inducible error-prone repair system. This similarity is further brought out by the observation that chloramphenicol has little or no effect on post-replication repair after UV irradiation. In E. coli, chloramphenicol has been reported to considerably inhibit post-replication repair in the wild type but not in the lexA mutant. Earlier work has suggested that most or all the mutations induced in H. influenzae by NC result from error-prone repair. Combined treatment with NC and either X-rays or UV shows that the NC error-prone repair system does not produce mutations from the lesions induced by these radiations even while it is producing them from its own lesions. It is concluded that the NC error-prone repair system or systems and the reclex error-prone system are different.

Mutagenesis of lambda phage: 5-bromouracil and hydroxylamine

Mutagenesis by 5-bromouracil of lambda phage to clear plaque formers does not depend on the recA function of the host E. coli cell or on the red function of the phage. Pretreatment of the host cells with ultraviolet light does not affect bromouracil mutagenesis of the adsorbed phage. Mutagenesis by hydroxlamine to clear plaque formers takes place at a high level in recA- host cells, and is not changed by preirradiation of of rec+ (wild type) hosts with ultraviolet light. Thus, bromouracil and hydroxylamine appear to mutate lambda phage by a process which differs from that responsible for ultraviolet mutagenesis. Two characteristics of bromouracil mutagenesis--the nonlinear dependence of the number of mutants on bromouracil incorporation, and a high frequency of heterozygotes--fit in with Rydberg's (1977) picture of bromouracil mutagenesis as a consequence of base mispairing, with mismatch repair removing the mutations at low incorporation of the analog.

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.

Fixation and loss of hydrazine-induced premutational damage in Haemophilus influenzae

Premutational damage induced in Haemophilus influenzae by hydrazine appears to be fixed as final mutation only at replication as judged by the transformation assay. Fixation at replication is independent of the rec1 gene, unlike the case with nitrosocarbaryl. Prior to replication premutational damage induced by hydrazine disappears by an unknown process that is not dependent on the presence of a pyrimidine dimer excision system nor on the rec1 gene. Hydrazine does not produce detectable single-strand breaks or alkali-labile sites in the treated DNA nor gaps in DNA newly synthesized after treatment. In these respects it also differs from nitroso compounds. It is concluded that hydrazine acts to produce and altered base, possibly N(4)-aminocytosine, that produces mutations by mispairing at replication rather than by error-prone repair.

Mutagenesis in bacteriophage T7. I. Chemically induced mutagenesis

The mutagenesis in phage T7 after MMS-, HNO2-, hydroxylamine-, 5-BUdR-, and 2-AP-treatment in relation to host controlled functions is investigated. There was no dependence of the induction of mutations on the character of the host strains (rec, hcr). A back mutation system (amber system) and a forward mutation system (host range system) have been used. Substances which cause mainly transitions from GC to AT do not lead or only rarely lead to reversions of the amber system; but chemicals producing transitions from AT to GC do so.

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.