A and alpha supernatant pretreatment of Saccharomyces cerevisiae cells affects both the kinetics and efficiency of mating

Sensory input attenuation allows predictive sexual response in yeast

Animals are known to adjust their sexual behaviour depending on mate competition. Here we report similar regulation for mating behaviour in a sexual unicellular eukaryote, the budding yeast Saccharomyces cerevisiae. We demonstrate that pheromone-based communication between the two mating types, coupled to input attenuation by recipient cells, enables yeast to robustly monitor relative mate abundance (sex ratio) within a mixed population and to adjust their commitment to sexual reproduction in proportion to their estimated chances of successful mating. The mechanism of sex-ratio sensing relies on the diffusible peptidase Bar1, which is known to degrade the pheromone signal produced by mating partners. We further show that such a response to sexual competition within a population can optimize the fitness trade-off between the costs and benefits of mating response induction. Our study thus provides an adaptive explanation for the known molecular mechanism of pheromone degradation in yeast.

Cells of the yeast Saccharomyces cerevisiae can mate with other cells of opposite mating type. Here, the authors show that the combination of a pheromone and a pheromone-degrading enzyme allows yeast cells to monitor relative mate abundance within a population and adjust their commitment to sexual reproduction.

A latent intron-encoded maturase is also an endonuclease needed for intron mobility

Some yeast mitochondrial introns encode proteins that promote either splicing (maturases) or intron propagation via gene conversion (the fit1 endonuclease). We surveyed introns in the coxl gene for their ability to engage in gene conversion and found that the group I intron, al4 alpha, was efficiently transmitted to genes lacking it. An endonucleolytic cleavage is detectable in recipient DNA molecules near the site of intron insertion in vivo and in vitro. Conversion is dependent on an intact al4 alpha open reading frame. This intron product is a latent maturase, but these data show that it is also a potent endonuclease involved in recombination. Dual function proteins that cleave DNA and facilitate RNA splicing may have played a pivotal role in the propagation and tolerance of introns.

Nonreciprocal exchange between alleles of the yeast mitochondrial 21S rRNA gene: kinetics and the involvement of a double-strand break

A 1.1 kb intron containing an open reading frame (ORF) in one allele (omega+) of the yeast mitochondrial 21S rRNA gene is nearly quantitatively inserted in crosses into a 21S rRNA allele lacking that intron (omega-). We have determined that this nonreciprocal exchange initiates soon after cells fuse to form zygotes and is complete by 10-16 hr after mating. We have discovered a unique in vivo double-strand cut in omega- mitochondrial DNA (mtDNA) at or near the site of intron insertion that is implicated in the process. Markers flanking the intron insertion site are coconverted with frequencies inversely proportional to their distance from that site. There is no net conversion of omega- to omega+ in crosses between petites retaining these alleles, nor do we observe the unique double-strand cut in the mtDNA from zygotes of such crosses. The data suggest that a translation product of the intron ORF is required for the double-strand cut and nonreciprocal recombination at omega.

Induction of yeast mating pheromone a-factor by alpha cells

Saccharomyces cerevisiae cells of a and alpha mating types constitutively secrete cell-specific peptide mating pheromones. a-Factor is secreted by a cells and acts on alpha cells, while alpha-factor is secreted by alpha cells and acts on a cells. Confirming preliminary studies, we demonstrate here that cultures of a cells contain higher than constitutive levels of a-factor activity when grown with alpha cells or alpha-factor. This induction of a-factor may result from increased synthesis or increased secretion of a-factor, as opposed to modification or stabilization of preexisting a-factor, as part of the a cell response to alpha-factor, as an a ste2 mutant (which cannot respond to alpha-factor) is not induced by alpha-factor. In mixed cultures inoculated with equal numbers of a cells and alpha cells, a cells predominate by stationary phase. Thus, a series of sequential interactions between a and alpha cells may be involved in establishing optimal hormone concentrations and cell ratios for conjugation.