Relationship between chromosomal alpha-specific suppressors of sir4-11 and polymorphism of the HMRa-bearing fragment.

In the course of studying sir4-11 which is responsible for the non-mating phenotype of asd-homothallism of Saccharomyces cerevisiae, we detected chromosomal alpha-specific suppressors of it. By examining the mating-type cassette constitution of two strains and the spore-clones derived from a diploid culture formed by a mass mating between these strains, we obtained the following results; (1) the HMRa-bearing HindIII-Bg/II restriction fragment was dimorphic, as judged by size, within each tetrad, (2) while one form was common to all tetrads, the other varied among tetrads, (3) spore-clones with the alpha-specific suppressor of sir4-11 had the variant forms while those without had the common form, and (4) while the two parents had the common form, each independent diploid clone had the common form plus a variant form. From these results, we conclude that the mating of cells in certain combinations induces a change of DNA structure at or near HMRa in a mating-pair specific manner and that the change makes HMRa non-derepressible or non-functional when derepressed.

Asd-homothallism of Saccharomyces cerevisiae: identification of asd1-1 as an allele of sir4 and detection of alpha-specific suppressors of it.

Asd-homothallism of Saccharomyces cerevisiae involves a life cycle characterized by a non-mating phenotype and endomitotic diploidization. The former trait is determined by a single mutation, asd1-1. This mutation was mapped between hom2 and lys4 on the right arm of chromosome IV and was complemented by the cloned SIR4 gene. Therefore, we conclude that asd1-1 is an allele of sir4-11 and renamed it sir4-11. Endomitotic diploidization of asd-homothallism is caused by the collaboration of three to four mutations including sir4-11. In the course of this study, we detected alpha-specific suppressors of sir4-11.

Interactions between chromosomal omnipotent suppressors and extrachromosomal effectors in Saccharomyces cerevisiae.

Chromosomal omnipotent suppressor mutations recovered in psi+ strains of Saccharomyces cerevisiae were brought into psi- cytoplasm. SUP46, SUP138 and SUP139 acted as dominant omnipotent suppressors in the psi- cytoplasm though their suppressor activity was substantially reduced. SUP46 and SUP138 conferred recessive thermosensitivity and antibiotic sensitivity in psi- cytoplasm as in psi+ cytoplasm. On the other hand, sup111 through sup115, which acted as recessive omnipotent suppressors in the psi+ cytoplasm, manifested no, or very low, suppressor activity in the psi- cytoplasm. They, however, still enhanced the efficiency of the SUP29 tRNA suppressor in psi- cytoplasm. A multicopy plasmid carrying the wild-type SUP35 gene enhanced the efficiency of sup111 in psi- cytoplasm.

"Alternative self-diploidization" or "ASD" homothallism in Saccharomyces cerevisiae: isolation of a mutant, nuclear-cytoplasmic interaction and endomitotic diploidization.

A mutant of Saccharomyces cerevisiae representing a novel life cycle, named "alternative self-diploidization" or "ASD" homothallism, was obtained fortuitously. In this life cycle, MAT alpha (or MATa) haplophase and MAT alpha/MAT alpha (or MATa/MATa) diplophase alternate. Germinated cells are haploid and mating. They soon become nonmating and sporogenous as they vegetatively grow. They sooner or later diploidize presumably via endomitosis. The diploid cells haploidize via normal meiosis. A single recessive nuclear mutation, named asd 1-1, is responsible for "ASD" homothallism. In the rho 0 cytoplasm, asd 1-1 cells mate even if at a low efficiency and fail to diploidize. Since pet mutations do not have such effects, we conclude that a certain mitochondrial function other than respiration is required for manifestation of "ASD" homothallism. That is, "ASD" homothallism is the result of some sort of nuclear-cytoplasmic interaction.

Two new loci that give rise to dominant omnipotent suppressors in Saccharomyces cerevisiae.

Ten dominant omnipotent suppressors of Saccharomyces cerevisiae, which were previously shown to be different from SUP46, have been examined. Nine are mapped in a region between lys5 and cyh2 on the left arm of chromosome VII. These suppressors, like SUP46, manifest sensitivity to increased temperature and the antibiotics paromomycin and hygromycin B. In addition, they have an identical action spectrum. These results strongly suggest that they are allelic to each other and they are designated SUP138. The tenth is mapped to a position between his1 and arg6 on the right arm of chromosome V. This suppressor, named SUP139, does not manifest temperature sensitivity nor antibiotic sensitivity. SUP139 and SUP138, which are clearly distinguished by means of action spectrum, act on much fewer nonsense mutations than SUP46. It is now clear that dominant omnipotent suppressors arising at a single locus are homogeneous and that their efficiency is locus-dependent. The order of efficiency is SUP46 greater than SUP138 greater than SUP139.

Cysteine biosynthesis in Saccharomyces cerevisiae: mutation that confers cystathionine beta-synthase deficiency.

The cys2-1 mutation of Saccharomyces cerevisiae was originally thought to confer cysteine dependence through a serine O-acetyltransferase deficiency. In this study, we show that cys2-1 strains lack not only serine O-acetyltransferase but also cystathionine beta-synthase. However, a prototrophic strain was found to be serine O-acetyltransferase deficient because of a mutation allelic to cys2-1. Moreover, revertants obtained from cys2-1 strains had serine O-acetyltransferase but not cystathionine beta-synthase, whereas transformants obtained by treating a cys2-1 strain with an S. cerevisiae genomic library had cystathionine beta-synthase but not serine O-acetyltransferase. From these observations, we conclude that cys2-1 (serine O-acetyltransferase deficiency) accompanies a very closely linked mutation that causes cystathionine beta-synthase deficiency and that these mutations together confer cysteine dependence. This newly identified mutation is named cys4-1. These results not only support our previous hypothesis that S. cerevisiae has two functional cysteine biosynthetic pathways but also reveal an interesting gene arrangement of the cysteine biosynthetic system.

Inheritance of chromosome length polymorphisms in Saccharomyces cerevisiae.

Although Saccharomyces cerevisiae strains generally have similar chromosomal band patterns as revealed by pulsed field gel electrophoresis, individual bands often move slightly differently from one strain to the other. Surveying strains from our stock collection, we found that nearly all the bands of a certain pair of strains differed in their mobility. Some of these chromosome length polymorphisms segregated in a 2:2 ratio, indicating that they resulted from single structural alterations (i.e. additions or deletions). One of these was mapped on the right arm of chromosome I. Others did not segregate in a simple 2:2 ratio. That is, there were progenies which had bands not present in either parent. We suggest that these new bands are the products of recombination between homologous chromosomes having two or more structural alterations.

UGA suppressors in Saccharomyces cerevisiae: allelism, action spectra and map positions.

Sixty independent UGA suppressors of Saccharomyces cerevisiae have been studied. They are dominant and are divided into 16 groups (loci) by recombination. Suppressors representing these loci are divided into two classes by action spectra; four in class 1 (a broad action spectrum) and 12 in class 2 (a narrow action spectrum). Class 1 suppressors are less frequent in terms of not only total number but also number per locus than class 2 suppressors, indicating difference in either or both mutation frequency and selective pressure between suppressors of the two classes. Two of the class 1 suppressors, SUP152 and SUP161, do not recombine with SUP28 and SUP33, leucine-inserting UAA suppressors, respectively, indicating that they are mutations in genes coding for tRNA(Leu)UUA. Of the remaining two class 1 suppressors, SUP160 which causes lethality in the psi+ cytoplasm is mapped on chromosome XV very close to the centromere, and SUP165 on the right arm of chromosome XIV 44 cM distal to lys9. Of the class 2 suppressors, ten do not recombine with one or another of previously known UGA suppressors. The remaining two class 2 suppressors, SUP154 and SUP155, are mapped on the left and right arms of chromosome VII, respectively.

Recessive nonsense suppressors in Saccharomyces cerevisiae: action spectra, complementation groups and map positions.

Three genes SUP111, SUP112 and SUP113 of Saccharomyces cerevisiae have been identified that can mutate to give recessive omnipotent nonsense suppressors. Alleles of these loci can also act as allosuppressors; that is, different phenotypes, due apparently to different efficiencies of suppression, can result from different alleles at a given locus. The SUP111, SUP112 and SUP113 loci map to the right arms of chromosomes VIII, VII and XIII, respectively.

Genetic mapping of leucine-inserting UAA suppressors in Saccharomyces cerevisiae.

Of S. cerevisiae UAA specific suppressors three leucine-insertors have been remaining unmapped. Mapping of them was attempted by using a set of eight strains that contained a total of 60 markers distributing over the entire genome; the markers were about 30 cM apart and they covered about 70% of the genome. Two supressors, SUP28 and SUP33, were localized on the right arm of chromosome XIV and on the left arm of chromosome XI, respectively. The other suppressor, SUP32, was not mapped, but its location was confined to a few regions in the genome. The present result and the previously available informations clearly show that the UAA suppressor loci are dispersedly distributing ovet the entire S. cerevisiae genome.