Urs Leupold

Concerted evolution of tRNA genes: Intergenic conversion among three unlinked serine tRNA genes in S. pombe

In many cases the multiple genes coding for one specific tRNA are dispersed throughout the genome. The members of such a gene family nevertheless maintain a common nucleotide sequence during evolution. A major mechanism contributing to this concerted evolution is intergenic conversion. Here we show that it occurs between three tRNA genes of related sequence residing on different chromosomes of Schizosaccharomyces pombe. Sequence analysis of converted genes indicates that blocks of a minimal length of 18-33 bp and of a maximal length of 190 bp can be transferred from one gene to the other. During meiosis the frequency of these transfers lies in the order of 10(-5) per progeny spore. Information transfer between any two members of the gene family occurs in both directions.

Sex appeal in fission yeast

SummaryUsing diploid strains of Schizosaccharomyces pombe, cytological evidence has been obtained which shows that not only h− cells (Fukui et al. 1986) but also h+ cells secrete a diffusible mating pheromone that attracts cells of the opposite mating type.

Genetic analysis of antisuppressor mutants in the fission yeast Schizosaccharomyces pombe

SummaryFourteen unlinked sin genes could be mutated to recessive antisuppressor alleles preventing the expression of suppressors in the fission yeast Schizosaccharomyces pombe. cyh1 alleles, resistant to the ribosomal inhibitor cycloheximide, also have some antisuppressor effect. The genetical and physiological characterization of these mutants is consistent with the hypothesis that they affect components of the messenger RNA translation machinery such as tRNA modifying enzymes or ribosomal proteins.

The genetic fine structure of nonsense suppressors in Schizosaccharomyces pombe : II. sup8 and sup10.

SummaryMeiotic fine-structure maps of two efficient UGA suppressors of Schizosaccharomyces pombe which are known (sup8-e) or inferred (sup10-e) to code for two leucine tRNAs carrying the mutant anticodon U*CA (Kohli et al. 1979, 1980a, b; Wetzel et al. 1979; Mao et al. 1981) are presented. In both cases, the recombination frequencies given by the primary site of the anticodon mutation fitwell into the map defined by the sites of a number of inactivating secondary mutations. This contrasts the corresponding situation found in the serine tRNA genes sup3 and sup9 where the anticodon site exhibits a specific marker effect which strongly increases recombination frequencies in crosses with all revertant sites, due to a decrease in the efficiency of excision repair of base-pair mismatches whenever the anticodon site is included in hybrid-DNA (Hofer et al. 1979; Munz and Leupold 1979; Thuriaux et al. 1980). A pronounced specific marker effect which leads to a several fold increase of the recombination frequencies over those expected is observed, however, at one of the secondary inactivating sites mapping in the leucine tRNA gene sup8.

Pheromone-induced meiosis in P-specific mutants of fission yeast

SummaryDiploid strains of Schizosaccharomyces pombe defective in P-specific pheromone production and response provided cytological evidence that the induction of meiosis as such (and not only conjugation) depends upon an initial stimulation by sexual pheromones.

Pheromone production and response in sterile mutants of fission yeast

SummaryGenetically heterothallic strains of various sterile mutants were assayed for residual production of the corresponding mating pheromone as well as responsivenes towards the opposite pheromone. No sexual activities were detected in ste11 strains (previously referred to as aff1 or steX, which we show are allelic), whilst the production of M factor was unaffected by ste1 to ste10 mutations. P factor production was still possible in class I ste mutants (ste5, ste6 and ste10), which also allow meiosis in diploid strains. With the exception of the leaky ste10-F23 mutant, no changes in cell morphology were induced by exposure to the opposite pheromone in the ste mutant strains.

Gene conversion in nonsense suppressors of Schizosaccharomyces pombe : II. Specific marker effects.

SummaryGene conversion and postmeiotic segregation patterns have been analysed at 14 mutant sites of sup3, sup8 and sup9 including 5 alleles with a strong marker effect on recombination frequencies in two-factor crosses. The total frequency of gene conversion and postmeiotic segregation tetrads is fairly constant within each gene, but may vary from one gene to another. About 97% of the conversion events are coconversions spanning the whole sup gene. Postmeiotic segregations are usually quite rare. None of the marker-effect alleles has an increased rate of hybrid DNA formation at the allele considered, as judged from the frequency of gene conversion and postmeiotic segregation in one-factor crosses. At least two of them, sup3-e and sup9-e, are associated with a high frequency of postmeiotic segregation indicating a poor repair of the corresponding base-pair mismatches. This is also observed in a two-factor cross and can account for the marker effect on recombination frequencies. The properties of a third marker effect allele, sup3-e,r10, are best explained by a higher probability of single site conversions as opposed to coconversions in two-factor crosses involving the mutant site r10.

Gene conversion in nonsense suppressors of Schizosaccharomyces pombe

SummaryThe frequency of gene conversion was assessed at the anticodon site of the sup3 gene of Schizosaccharomyces pombe. In order to detect a possible influence of the genetic background on the relative frequency of post-meiotic segregations amongst conversions, three similar crosses were analyzed which differed as follows: In cross I the two parents were derived by spontaneous mutation from one and the same strain. The heterogeneity of the genetic background between these two parent strains is assumed to be at a minimum level. In cross II the crossing partners were strains of the Bernese stock collection and differ probably in their genetic background to some extent. Last, in cross III, one of the parent strains was ten times repeatedly mutagenized with nitrosoguanidine in order to introduce cryptic mutations in the genome. A maximum degree of background heterogeneity between parents is expected for this cross. Neither the total conversion frequency nor the frequency of post-meiotic segregations amongst conversions were found to be significantly different in these three crosses.In addition, no effect of the radiation-sensitive mutation, rad2-44, and of two mutator mutations, mut1-4 and mut2-9, on the conversion pattern could be detected.

Mitotic recombination between dispersed but related rRNA genes of Schizosaccharomyces pombe generates a reciprocal translocation

SummaryRecombination between dispersed yet related serine tRNA genes of Schizosaccharomyces pombe does occur during mitosis but it is approximately three orders of magnitude less frequent than in meiosis. Two mitotic events have been studied in detail. In the first, a sequence of at least 18 nucleotides has been transferred from the donor sup3 gene on the right arm of chromosome I to the related acceptor gene sup12 on the left arm of the same chromosome, thereby leading to the simultaneous change of 8 bp in the acceptor gene. This event must be explained in terms of recombination rather than mutation. It is assumed that it represents mitotic gene conversion, although it was not possible to demonstrate that the donor gene had emerged unchanged from the event. The second case reflects an interaction between sup9 on chromosome III and sup3 on chromosome I. Genetic and physical analysis allows this event to be described as mitotic gene conversion associated with crossingover. The result of this event is a reciprocal translocation. No further chromosomal aberrations were found among an additional 700 potential intergenic convertants tested. Thus intergenic conversion is much less frequently associated with crossingover than allelic conversion. However, the rare intergenic conversion events associated with crossingover provide a molecular mechanism for chromosomal rearrangements.

Sterile UGA nonsense mutants of fission yeast

SummaryEight sterile mutants, which regain their fertility upon reactivation of an inactivated UGA suppressor allele of the serine tRNA gene sup3, are shown to carry UGA nonsense alleles of two established ste genes, ste1 (one mutant) and ste6 (two mutants), and of two novel genes, ste9 (four mutants) and ste10 (one leaky mutant of ras1-/ste5-like cell morphology). The mutant alleles of ste1 and ste9 lead to a defect in both conjugation and meiosis, whereas those of ste6 and ste10 affect mating only. Two of the four genes map to chromosome I, ste1 in the left arm 6 cM distal of ura1, and ste9 in the right arm 3 cM distal of ade2. The ste10 and ste6 genes are located in the right arms of chromosomes II and III, respectively, the former 4 cM distal of trp1 and the latter 1 cM proximal or distal of trp3.