Dispersed repetitive DNA sequences from yeast (Saccharomyces cerevisiae) nuclear DNA have been isolated as molecular hybrids in lambdagt. Related S. cerevisiae strains show marked alterations in the size of the restriction fragments containing these repetitive DNAs. "Ty1" is one such family of repeated sequences in yeast and consists of a 5.6 kilobase (kb) sequence including a noninverted 0.25 kb sequence of another repetitious family, "delta", on each end. There are about 35 copies of Ty1 and at least 100 copies of delta (not always associated with Ty1) in the haploid genome. A few Ty1 elements are tandem and/or circular, but most are disperse and show (along with delta) some sequence divergence between repeat units. Sequence alterations involving Ty1 elements have been found during the continual propagation of a single yeast clone over the course of a month. One region with a large number of delta sequences (SUP4) also shows a high frequency of sequence alterations when different strains are compared. One of the differences between two such strains involves the presence or absence of a Ty1 element. The novel joint is at one inverted pair of delta sequences.

Evidence for transposition of dispersed repetitive DNA families in yeast [Saccharomyces cerevisiae]

Publisher Summary Transposable elements are the DNA sequences that move to new genomic locations at a much higher rate than that of the bulk of the cellular DNA. The ability to cause deletions or chromosomal rearrangements is characteristic of these elements, and many have been shown to affect the expression of chromosomal genes by inserting adjacent to or into these genes. Saccharomyces cerevisiae is very useful as a eukaryotic model system for molecular biologists. Features of a yeast system that make this organism advantageous include the ease of growth, small genome size, the availability of selectable markers, the ease of genetic manipulation, and the availability of techniques for transformation with exogenous DNA. Yeast transposable element (Ty) is a family of disperse, repetitive DNA sequences, each of which is homologous to the original Ty 1 sequence. Yeast has quite a small genome, as compared to other eukaryotes, and much less repeated DNA. Transposable elements in many systems appear to cause chromosomal rearrangements, such as deletions, inversions, and translocations. Several types of chromosomal rearrangements have been reported to be associated with yeast Ty elements or delta sequences.

Transposable elements in yeast.

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.

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

CHAPTER 7 – Transposable Elements in Yeast

https://www.cell.com/cell/pdf/0092-8674(81)90209-9.pdf

Ty elements are involved in the formation of deletions in DEL1 strains of saccharomyces cerevisiae

The DEL1 mutator in Saccharomyces cerevisiae leads to the formation of deletions adjacent to itself (Liebman et al. 1979). Here we show that the frequency of these DEL1-promoted deletions is not altered by the presence of the recombination-deficient mutation, rad52-1. This indicates that generalized recombination is not required for the formation of deletions in DEL1 yeast strains.

Karen M. Downs

Papers

The RAD52 gene is not required for the function of the DEL1 mutator gene in Saccharomyces cerevisiae.