Heat shock-induced degradation of Msn2p, a Saccharomyces cerevisiae transcription factor, occurs in the nucleus.

In the yeast Saccharomyces cerevisiae, the zinc finger transcription factor Msn2p is a central component of the general stress response. It is activated in response to a wide variety of environmental changes, including physicochemical stresses as well as nutritional starvation, and induces the expression of a large set of genes required for cellular adaptation. The transcriptional activity of Msn2p in response to stresses is transient, and must therefore be strictly controlled. It is mainly regulated by reversible translocation from the cytoplasm to the nucleus upon the onset of stress, under the control of the cAMP-APK and the TOR pathways. In this report, we describe a new level of control: heat shock-induced degradation of Msn2p by the 26S proteasome. This degradation occurs in the nucleus and is further enhanced when Msn2p is fully active. Moreover, we show that the cyclin-dependent protein kinase Srb10p, a component of the transcription machinery, plays a role in the enhanced degradation of Msn2p upon heat shock. These findings provide new insights into the mechanisms by which Msn2p is transiently activated in response to stress.

Mutations in mating-type genes of the heterothallic fungus Podospora anserina lead to self-fertility.

The heterothallic fungus Podospora anserina has two mating-type alleles termed mat+ and mat-. The mat+ sequence contains one gene, FPR1, while mat- contains three genes: FMR1, SMR1, and SMR2. FPR1 and FMR1 are required for fertilization, which is followed by mitotic divisions of the two parental nuclei inside the female organ. This leads to the formation of plurinucleate cells containing a mixture of parental mat+ and mat- nuclei. Further development requires a recognition between mat+ and mat- nuclei before migration of the mat+/mat- pairs into specialized hyphae in which karyogamy, meiosis, and ascospore formation take place. FPR1, FMR1, and SMR2 control this internuclear recognition step. Initial development of the dikaryotic stage is supposed to require SMR1; disruption of SMR1 results in barren perithecia. In a systematic search for suppressors restoring fertility, we isolated 15 suppressors-all of them mutations in the mating-type genes. These fmr1, smr2, and fpr1 mutants, as well as the strains disrupted for FMR1, SMR2, and FPR1, are weakly self-fertile. They are able to act as the male partner on a strain of the same mating type and give a mixture of biparental and uniparental progeny when crossed with a wild-type strain of opposite mating type. These observations lead us to propose that SMR2, FMR1, and FPR1 act as activators and repressors of fertilization and internuclear recognition functions.

pah1: a homeobox gene involved in hyphal morphology and microconidiogenesis in the filamentous ascomycete Podospora anserina.

Homeobox-containing genes are widely described among eukaryotic species other than filamentous ascomycetes. We describe here the isolation and characterization of the first homeobox gene (pah1) identified in a filamentous ascomycete. It encodes a putative protein of 610 amino acids containing a typical homeodomain with 60 amino acids. Deletion of the pah1 gene enhances the number of male gametes (microconidia), whereas overexpression of pah1 results in a decrease in microconidia. These results led us to suppose that pah1 may be a repressor of genes involved in the microconidiation process. Moreover, pah1 is involved in hyphal branching and possibly in the development of female organs.