Mother cell-specific HO expression in budding yeast depends on the unconventional myosin myo4p and other cytoplasmic proteins

Certain cell types give rise to progeny that adopt different patterns of gene expression in the absence of any differences in their environment. Cells of budding yeast give birth to mother and daughter cells that differ in that only mother cells express the HO endonuclease gene and thereby switch mating types. We describe the identification of five genes, called SHE1-SHE5, that encode cytoplasmic proteins required for mother-specific HO expression. She1p, which is identical to the minimyosin Myo4p, and She3p are not, however, mother-specific proteins. On the contrary, they accumulate in growing buds. She proteins might be required for the transport of factors that promote HO repression from the mother cell into its bud. In an accompanying paper, we show that SHE genes are needed for the accumulation in daughter nuclei of Ash1p, a repressor of HO.

Destruction of the CDC28/CLB mitotic kinase is not required for the metaphase to anaphase transition in budding yeast

It is widely assumed that degradation of mitotic cyclins causes a decrease in mitotic cdc2/CDC28 kinase activity and thereby triggers the metaphase to anaphase transition. Two observations made on the budding yeast Saccharomyces cerevisiae are inconsistent with this scenario: (i) anaphase occurs in the presence of high levels of kinase in cdc15 mutants and (ii) overproduction of a B-type mitotic cyclin causes arrest not in metaphase as previously reported but in telophase. Kinase destruction is therefore implicated in the exit from mitosis rather than the entry into anaphase. The behaviour of esp1 mutants shows in addition that kinase destruction can occur in the absence of anaphase completion. The execution of anaphase and the destruction of CDC28 kinase activity therefore appear to take place independently of one another.

Specific binding sites in the alcR and alcA promoters of the ethanol regulon for the CREA repressor mediating carbon catabolite repression in Aspergillus nidulans

The CREA repressor responsible for carbon catabolite repression in Aspergillus nidulans represses the transcription of the ethanol regulon. The N-terminal part of the CREA protein encompassing the two zinc fingers (C2H2 class family) and an alanine-rich region was expressed in Escherichia coli as a fusion protein with glutathione-S-transferase. Our results show that CREA is a DNA-binding protein able to bind to the promoters of both the specific trans-acting gene, alcR, and of the structural gene, alcA, encoding the alcohol dehydrogenase I. DNase I protection footprinting experiments revealed several specific binding sites in the alcR and in the alcA promoters having the consensus sequence 5'-G/CPyGGGG-3'. The disruption of one of these CREA-binding sites in the alcR promoter overlapping the induction target for the trans-activator ALCR results in a partially derepressed alc phenotype and derepressed alcR transcription, showing that this binding site is functional in vivo. Our data suggest that CREA represses the ethanol regulon by a double lock mechanism repressing both the trans-acting gene, alcR, and the structural gene, alcA.

SWI5 instability may be necessary but is not sufficient for asymmetric HO expression in yeast

Homothallic haploid yeast cells divide to produce a mother cell that switches mating type and a daughter cell that does not. This pattern is the result of HO endonuclease transcription exclusively in mother cells, and there only transiently in late G1 as cells undergo Start. SWI5 encodes an HO transcription factor that is expressed during the S, G2, and M phases of the cell cycle. The lack of synthesis of SWI5 during G1 is essential to prevent HO transcription in daughter cells. Thus, HO must be activated by SWI5 protein synthesized in the previous cell cycle if it is to be properly regulated. SWI5 is inherited by both mother and daughter cells, and we show here that most of it is rapidly degraded during early G1. More stable mutant SWI5 proteins cause daughter cells to switch mating type, suggesting that SWI5 destruction is necessary to prevent HO expression in daughters. We show further that mother cells can still express HO when stimulated to undergo Start after arrest in early G1 for several hours. We propose that a small fraction of the SWI5 protein inherited by mother cells is extremely stable and that the crucial difference between mothers and daughters with regard to HO transcription is their differential ability to sequester SWI5 in a stable form, possibly as a component of transcription complexes on the HO promoter.