[High frequency reversion of a spontaneous mutant in Ascobolus immersus].
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A spontaneous spore-colour mutation located in the gene b2 of A. immersus gives a high frequency of reversion. 8 distinct revertant phenotypes have been defined. The mutations corresponding to these revertants are located at the same site or very near the original mutation. The hypothesis that the mutation by itself is responsable for the observed reversions is advanced.Keywords:
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Experiment has been carried out using UV-sensitive mutant of uvs 1 of Saccharomyces cerevisiae, presumably lacking ability of dark repair, to investigate genetic nature of UV-induced premutational damages leading to three-different types of mutation under the dark and illuminated conditions. Yields of these types of mutation in uvs 1 are markedly higher than those in wild type at equal doses. At the dark condition, logarithmically plotted dose-response curves of true-back mutations from arg 4-17, his 5-2 and lys 1-1 induced in uvs 1 consist of two straight lines. Slopes of the curves at high doses are more steeper than those at low doses and almost the same as those in wild type in all the dose range. The curves for induced frequency of super-suppressor mutation and that of back mutation from his 1-1, i.e., addition-deletion type mutation consists of one component, and their slopes are nearly equal for both uvs 1 and wild-type strains. Furthermore, ratios of true back mutation yields between uvs 1 and wild-type strains at equal doses are greater than those of super-suppressor mutations or reversion of addition-deletion type. To account for these results it is postulated that premutational damage leading to true-back mutation is preferentially repairable compared with damages responsible for the other types of mutation. Molecular natures of the premutational damages leading to the three types of mutation are discussed.
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Revertants obtained from three threonine-requiring mutants of Bacillus subtilis were studied. The revertants from mutants 347 and 350 did not form normal colonies at 37° while they did at 50° (temperature-dependent), and the revertants from mutant 530 did not form colonies at 50° while they did at 37° (temperature-sensitive). By genetic analysis, one reversion observed in 350 was attributed to a mutation occurring in the same cistron, and one reversion observed in 530 was attributed to a suppressor mutation occurring at a map position of 0.64. Mechanism of reversion was discussed.
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Abstract We previously identified Caenorhabditis elegans mutants in which certain of the six vulval precursor cells adopt fates normally expressed by other vulval precursor cells. These mutants define genes that appear to function in the response to an intercellular signal that induces vulval development. The multivulva (Muv) phenotype of one such mutant, CB1322, results from an interaction between two unlinked mutations, lin-8(n111) II and lin-9(n112) III. In this paper, we identify 18 new mutations, which are alleles of eight genes, that interact with either lin-8(n111) or lin-9(n112) to generate a Muv phenotype. None of these 20 mutations alone causes any vulval cell lineage defects. The "silent Muv" mutations fall into two classes; hermaphrodites carrying a mutation of each class are Muv, while hermaphrodites carrying two mutations of the same class have a wild-type vulval phenotype. Our results indicate that the Muv phenotype of these mutants results from defects in two functionally-redundant pathways, thereby demonstrating that redundancy can occur at the level of gene pathways as well as at the level of gene families.
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This paper describes a mutator system in the nematode Caenorhabditis elegans var. Bergerac for the gene unc-22. Of nine C. elegans and two C. briggsae strains tested only the Bergerac BO strain yielded mutant animals at a high frequency and the unc-22 IV gene is a preferred mutational target. The forward spontaneous mutation frequency at the unc-22 locus in Bergerac BO is about 1 X 10(-4), and most of these spontaneous unc-22 mutations revert at frequencies between 2 X 10(-3) and 2 X 10(-4). Both the forward mutation frequency and the reversion frequency are sensitive to genetic background. Spontaneous unc-22 mutations derived in a Bergerac background and placed in a primarily Bristol background revert at frequencies of less than 10(-6). When reintroduced into a Bergerac/Bristol hybrid background the mutations once again become unstable. The mutator activity could not be localized to a discrete site in the Bergerac genome. Nor did mutator activity require the Bergerac unc-22 gene as a target since the Bristol unc-22 homolog placed in a Bergerac background also showed high mutation frequency. Intragenic mapping of two spontaneous unc-22 alleles, st136 and st137, place both mutations in the central region of the known unc-22 map. However, these mutations probably recombine with one another, suggesting that the unstable mutations can occur in more than one site in unc-22. Examination of the phenotypic effect of these mutations on muscle structure indicates that they are less severe in their effect than a known amber allele. We suggest that this mutator system is polygenic and dispersed over the nematode genome and could represent activity of the transposable element Tc1.
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A compensatory mutation occurs when the fitness loss caused by one mutation is remedied by its epistatic interaction with a second mutation at a different site in the genome. This poorly understood biological phenomenon has important implications, not only for the evolutionary consequences of mutation, but also for the genetic complexity of adaptation. We have carried out the first direct experimental measurement of the average rate of compensatory mutation. An arbitrary selection of 21 missense substitutions with deleterious effects on fitness was introduced by site-directed mutagenesis into the bacteriophage phiX174. For each deleterious mutation, we evolved 8-16 replicate populations to determine the frequency at which a compensatory mutation, instead of the back mutation, was acquired to recover fitness. The overall frequency of compensatory mutation was approximately 70%. Deleterious mutations that were more severe were significantly more likely to be compensated for. Furthermore, experimental reversion of deleterious mutations revealed that compensatory mutations have deleterious effects in a wild-type background. A large diversity of intragenic compensatory mutations was identified from sequencing fitness-recovering genotypes. Subsequent analyses of intragenic mutation diversity revealed a significant degree of clustering around the deleterious mutation in the linear sequence and also within folded protein structures. Moreover, a likelihood analysis of mutation diversity predicts that, on average, a deleterious mutation can be compensated by about nine different intragenic compensatory mutations. We estimate that about half of all compensatory mutations are located extragenically in this organism.
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Escherichia coli mutant N4316 is temperature sensitive and exhibits temperature-dependent suppression. These phenotypes are due to separate genes, as shown by reversion and mapping studies. The suppressor mutation was mapped and lies near argF .
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Collections of mutants usually contain more mutants bearing multiple mutations than expected from the mutant frequency and a random distribution of mutations. This excess is seen in a variety of organisms and also after DNA synthesis in vitro. The excess is unlikely to originate in mutator mutants but rather from transient hypermutability resulting from a perturbation of one of the many transactions that maintain genetic fidelity. The multiple mutations are sometimes clustered and sometimes randomly distributed. We model some spectra as populations comprising a majority with a low mutation frequency and a minority with a high mutation frequency. In the case of mutants produced in vitro by a bacteriophage RB69 mutator DNA polymerase, mutants with two mutations are in approximately 10-fold excess and mutants with three mutations are in even greater excess. However, phenotypically undetectable mutations seen only as hitchhikers with detectable mutations are approximately 5-fold more frequent than mutants bearing detectable mutations, indicating that they arose in a subpopulation with a higher mutation frequency. Excess multiple mutations may contribute critically to carcinogenesis and to adaptive mutation, including the adaptations of pathogens as they move from host to host. In the case of the rapidly mutating riboviruses, the viral population appears to be composed of a majority with a mutation frequency substantially lower than the average and a minority with a huge mutational load.
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An analysis of revertants of missense mutants in phage P22 has shown: (i) New temperature-sensitive (TS) and cold-sensitive (CS) phenotypes are often acquired concomitant with reversion. (ii) In many cases, these new phenotypes are due to second-site mutations (suppressors) that correct the original defect. (iii) Sometimes the suppressor mutation is not in the same gene as the original mutation. (iv) Extragenic suppressors are almost always in genes whose products are known to interact physically with the original gene products. (v) The suppressor mutations typically retain their TS or CS phenotypes when crossed into wild-type genetic backgrounds. (vi) Some TS and CS mutants derived by reversion can themselves be reverted to produce additional mutations. We have shown that genetic reversion of missense mutants can be of value in producing new temperature-sensitive and cold-sensitive mutations affecting related functions. We suggest that our approach can be extended to organisms with large genomes.
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