A long region upstream of the IME1 gene regulates meiosis in yeast.

Meiosis and sporulation in yeast are subject to two types of regulation. The first depends on environmental conditions. The second depends on a genetic pathway which involves the control of the positive regulatory gene IME1 by RME1, which is in turn controlled by the MAT locus. The presence of IME1 on a multicopy plasmid enables cells to undergo meiosis regardless of their genotype at MAT or RME1. We show here that a multicopy plasmid carrying IME1 also enables meiosis, regardless of the environment. Therefore, both kinds of regulation appear to act through IME1. Furthermore, the behavior of multicopy plasmids carrying various segments from the IME1 region suggests that the region upstream of IME1 contains both positive and negative regulatory sites. Control of IME1 by the environment and by the MAT pathway both act through negative regulatory sites.

The sporulation capable (sca) mutation of Saccharomyces cerevisiae is an allele of the SIR2 gene.

We have used the special properties of the spo13-1 mutation in order to study the regulation of yeast meiosis by the mating type loci. We have found that both the rme1-1 mutation and the sca mutation allow haploid meiosis in spo13-1 strains. Therefore, haploid meiosis is regulated in the same manner as diploid meiosis. Unlike rme1-1, the sca mutation allows meiosis through derepression of the silent mating type cassettes; sca strains can sporulate only because they express both MATa and MAT alpha information. We have found further that sca is an allele of SIR2, one of the genes involved in repression of the silent cassettes. Therefore, the RME1 gene is the only known candidate for a master negative regulator through which the MAT locus controls meiosis.

Coincident gene conversion events in yeast that involve a large insertion.

In yeast, spontaneous gene conversion events involving sites that are far apart (16 cM) occur 1000 times more frequently in mitotic cells than is expected for two independent acts of recombination. It has been proposed that a major portion of these could be due to a long, continuous heteroduplex intermediate. We have examined this possibility in further detail by introducing, via transformation, a large plasmid insertion between the LEU1 and TRP5 loci and studying its behavior among coincident convertants involving the flanking sites. Among such convertants, there is frequent loss of the plasmid when it is present in hemizygous or homozygous configuration. Our results could support the long heteroduplex model for coincident recombination events, but only if novel assumptions regarding the formation and fate of mismatched DNA are made. Therefore, an alternative model that proposes multiple, concerted recombination events is discussed.

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum.

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.