Nuclear mutations in Saccharomyces cerevisiae which increase the spontaneous mutation frequency in mitochondrial DNA

The evolution of epidemic Vibrio cholerae strains

The emergence of the novel Vibrio cholerae strain, O139 Bengal, which caused a large epidemic in Southeast Asia, underlines the adaptability of pathogenic microorganisms. Recent studies reveal that horizontal transfer of cell-wall polysaccharide genes played a central role in the emergence of this strain and that its genesis may not be as unique as initially believed.

Genetic analysis in vibrio

Bacteria of the genus Vibrio are remarkably diverse, and until recently the methodology for genetic analysis consisted of a patchwork of different approaches, many of which were narrowly applicable to a single species. The invention of the recombinant DNA technology and the subsequent innovations in transposon mutagenesis and in transductive and conjugative gene transfer techniques have led to the development of very powerful and general strategies for genetic analysis of species of Vibrio. The striking synergy of combining recombinant DNA, transposon, and gene transfer methods is particularly evident in the construction of transposons which generate gene fusions and of broad host range plasmids which deliver transposons and mutated genes and which mobilize chromosomes. With such tools it should be possible to perform advanced genetic analysis on the many undomesticated species of Vibrio still to be explored.

The absence of introns in yeast mitochondria does not abolish mitochondrial recombination

The respiratory competency of a yeast strain devoid of mitochondrial introns is quite normal. However, it may be asked whether intron-encoded proteins participate in metabolisms other than those of mitochondrial introns. Using strains without mitochondrial introns we have answered two questions. The first was: does the absence of intron-encoded proteins abolish mitochondrial recombination? The second was: do mitochondrial introns and intron-encoded proteins play a part in mitochondrial DNA rearrangements induced by ethidium bromide (rho- production)? We have shown that the introns and intron-encoded proteins are not essential components of either phenomenon.

Isolation and initial characterization of a Schizosaccharomyces pombe mutant exhibiting temperature-dependent radiation sensitivity due to a mutation in a previously unidentified rad locus

We have isolated a mutant of the yeast Schizosaccharomyces pombe which exhibits sensitivity to UV light when grown at either 30 degrees or 37 degrees C, as compared to the parental wild-type strain. This increased sensitivity is more pronounced when cells are grown at 37 degrees C. The mutant is also sensitive to 18 MeV electrons at the high temperature. Tetrad analysis of spores generated by crossing the mutant and a Rad+ strain revealed that sensitivity to both types of radiation cosegregate 2:2, relative to wild-type resistance, indicating that a single altered chromosomal locus is responsible for the radiation sensitivities observed. In addition, analysis of spores resulting from crosses between the mutant and all other known S. pombe rad mutants indicates that the temperature-dependent sensitivity described in this report is mediated by a mutation in a previously unidentified rad locus.

Repair properties in yeast mitochondrial DNA mutators

After ethylmethanesulfonate mutagenesis of the strain Saccharomyces cerevisiae D273-10B, out of 100,000 survivors, 1,000 were selected for their high production of petite mutants at 36 degrees C. Among these 1,000 mutators, 5 also showed an increased frequency of spontaneous point mutations measured at 25 degrees C. Further analysis revealed that in all mutators, except 2, petite accumulation proceeded at 25 degrees C as well as 36 degrees C. In these 2 mutants, the production of petite mutants was much higher at 36 degrees C than at 35 degrees C. In one of them, however, the mutator and the thermosensitive petite phenotypes were due to mutations in two unlinked nuclear genes. In the other mutants, both traits were the result of a mutation in a single nuclear gene. The mutators fell into three complementation groups (tpm1, tpm2, mup1). No complementation was observed between tpm1 mutants and the gam4 mutant previously described by Foury and Goffeau (1979). From the latter and the present works, only four complementation groups (gam1, gam2, gam4 or tpm1, mup1) have been identified and it is likely that the number of genes controlling specifically the spontaneous mutability of the mtDNA is low. The mutators exhibited a variety of responses to damaging agents such as UV light and ethidium bromide; especially in a representative mutant from the complementation group tpm1, the induction of rho- mutants was sensitive to UV light and resistant to ethidium bromide.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

Vectors for transposon mutagenesis of non-enteric bacteria

We have constructed a series of transposon delivery vectors derived from pRK2013. Since pRK2013 has a broad host range transfer system and a ColE1 replicon, it can be transferred to, but not replicated in, many non-enteric gram-negative bacteria. Thus pRK2013 provides an effective mechanism for the transient introduction of a transposon. Delivery vectors containing Tn7 (tmp str), Tn10 (tet), Tn10 HH104 (tet), or Tn5-132 (tet) have been constructed. When transposition in Caulobacter crescentus was examined, both Tn7 and Tn5-132 were found to transpose efficiently. In contrast, although the antibiotic resistances of Tn10 and Tn501 (mer) were expressed in C. crescentus, no transposition was observed with either transposon. However, transposition of Tn10 from the Tn10 vectors did occur in Acinetobacter calcoaceticus, and transposition of Tn501 from pMD100 has been demonstrated in Rhizobium japonicum (Bullerjahn and Benzinger 1984). Thus, transposon-host interactions play an important role in the determination of whether a particular transposon can transpose in a given host. Furthermore, the results with C. crescentus indicate that there must be different requirements for host interactions for Tn10 and Tn501 than for Tn5 and Tn7.

Yeast mutants affecting the spontaneous mutation frequency on both the mitochondrial and nuclear genomes

We have previously described two complementation groups of mutants affecting the spontaneous mutation frequency on the mitochondrial genome (Johnston, 1979). In a further search for such mutants the majority isolated fell into one of these groups, 3 into group I and 14 into group II. There are now a total of 12 alleles in the first group and 19 in the second complementation group, suggesting that these are the major classes of mutants affecting spontaneous mutation frequency in the mitochondrion. However, this search also identified a third complementation group, consisting of 2 mutants, which, like the existing groups, is recessive and is coded on the nuclear genome. In contrast to complementation groups I and II, these new mutants have no effect on spontaneous petite mutagenesis and they also increase the spontaneous mutation frequency on the nuclear genome. This nuclear mutation activity may be novel, as it complemented the existing nuclear mutators mut1-mut10. None of the three complementation groups has any detectable phenotype, other than the mutator activity.

Transposition of Tn 1 to the Rhizobium meliloti genome

A derivative of the IncP1 plasmid RP4, carrying the thermoinducible prophage Mucts62, was obtained in Escherichia coli K12 J53 (RP4). It was impossible to maintain the hybrid plasmid RP4::Mucts62 in Rhizobium meliloti GR4. Thus, it was used as a vehicle for introducing the ampicillin-resistant transposon Tn1 into the R. meliloti genome. Transposition of Tn1 did not generate auxotrophic strains, suggesting that the insertion of Tn1 into the R. meliloti genome was relatively specific. Two chromosomal hot spots for Tn1 insertion were identified by cotransductional analysis, after general transduction by phage DF2. Plasmid-curing experiments, carried out by heat treatment, revealed that symbiotic plasmid(s) also contain at least one site for Tn1 insertion.