The genetics of recombination / D.G. Catcheside.
- David Guthrie Catcheside
- Date:
- [1977]
Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
Credit: The genetics of recombination / D.G. Catcheside. Source: Wellcome Collection.
81/192 page 67
![4.1 Genes of general effect 67 been subjected to mutagen. They were then replicated to solid synthetic medium lacking arginine and then irradiated with X-rays (2.5 X 10^ rad). The dose of irradiation is sufficient to produce about 20 prototrophs per colony in normal strains. Colonies without reversions were saved, from the original plates, as potentially deficient in recombination and tested further. Ten mutants were found and seven were studied in detail. They showed, by complementation and other tests of allelism, a minimum of four loci: reel, rec2, rec3 and rec4. Two mutants are sensitive (killed more readily) to X-rays (one being rec2), one is sensitive to X-rays and ultraviolet light, while the other four are insensitive to UV and to X-rays. Besides the lack of induction of mitotic recombination by X-rays all, except rec2, show a similar lack of induction by UV. There are differences between these mutants and the wild type with respect to meiotic recombination (Rodarte-Ramón, 1972). Meiosis is abortive in homozygotes гесЗ and nearly so in rec2. In reel, spontaneous and induced allelic recombination at the arg4 locus at meiosis is about equal to the control, but in rec4 it is depressed. Non-allelic recombination appears to be normal in reel and rec4 strains. Evidently some, but not all, steps in mitotic and meiotic recombination are in common. Lemontt (1971a) has selected mutants at three loci {revi, rev2 and rev3) that are defective in induced mutation. One locus {rev2) is the same as a previously known locus [radS] at which mutation leads to sensitivity to ultraviolet light. Recombination at meiosis in rev homozygotes occurs (Lemontt, 1971b) at control frequencies between leul and trp5 and between arg4-6 and arg4-17. The frequency of mitotic recombination induced by UV or by X-rays, measured for the centromere to ade2 segment and for arg4-6¡arg4-17 increased more sharply with radiation dose in rev rev diploids than in + + ones. Mitotic recombination, although induced by UV damage, is not correlated with UV mutagenesis. Spontaneous mutability is also under genetic control, von Borstel, Cain and Steinberg (1971) showed by a specially designed fluctuation test that spontaneous mutation rates were enhanced in three different radiation sensitive mutant strains. However, the enhancement was specific to particular kinds of mutation. Three classes of mutant have been distinguished in this work: (1) super suppressor genes of class I (Hawthorne and Mortimer, 1968) that generate mutants which modify tyrosine tRNA so that it recognizes the ochre codon and so suppress ochre nonsense mutants; (2) the nonsense ochre mutants; and (3) frame shift mutants. Two of the radiation sensitive mutants {jadis and radSS) confer enhanced mutation rates on the super suppressor genes of class I while rad2 depresses their mutation rate. The radiation sensitive mutant radSl enhances the mutation rate of the ochre nonsense locus as well as of the suppressors, but much more in the latter. Moreover, radlS and radSl also increase mutation rates of a frame shift mutant. It is believed that the mutators act by affecting the addition or deletion of bases in DNA. The connection with recombination is that two of the radiation sensitive mutants are defective in sporulation. The](https://iiif.wellcomecollection.org/image/b18025560_0082.JP2/full/800%2C/0/default.jpg)
