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Regulation of mating type switching in Kluyveromyces lactis by the RAS/cAMP pathway and the transcription factor Msn2
Stockholm University, Faculty of Science, The Wenner-Gren Institute , Developmental Biology. (Stefan Åström)
Stockholm University, Faculty of Science, The Wenner-Gren Institute , Developmental Biology. (Stefan Åström)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We explored the regulation of mating type switching in Kluyveromyces lactis. Using an assay dependent on loss of a URA3 gene inserted into the MATa locus, we determined that the switching rate of a wild type strain grown in rich media was ~6X10-4 events/generation. In a genetic selection for identifying strains with increased switching rates, we found a strain with an insertion in the K. lactis RAS1 gene, encoding a small GTPase with a central role in growth regulation. Compromised Ras1 function leads to a lower cAMP level suggesting a role for cAMP in promoting switching. Consistent with this idea, a strain lacking the PDE2 gene, which encodes an enzyme that degrades cAMP, resulted in decreased switching rates. To explore how cAMP regulated switching, we investigated the transcription of the MTS1 gene, encoding an inducer of switching. The ras1 mutant strain contained 20-fold higher levels of the MTS1 mRNA compared to wild type, but in the pde2 mutant strain MTS1 transcription was repressed 5-fold. In addition, strains lacking the MSN2 gene, which encodes a transcription factor that binds the stress response element (STRE), expressed less MTS1 mRNA and had decreased switching rates. We suggest a model in which nutrient limitation induces switching through cAMP and Msn2-dependent transcriptional induction of the MTS1 gene.

Keyword [en]
mating type switch, K. lactis, Ras, cAMP Msn2, Pde2, Mts1
National Category
Developmental Biology
Research subject
Zoological Developmental Biology
Identifiers
URN: urn:nbn:se:su:diva-42013OAI: oai:DiVA.org:su-42013DiVA: diva2:343518
Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2010-08-16Bibliographically approved
In thesis
1. Mating type switching and transcriptional silencing in Kluyveromyces lactis
Open this publication in new window or tab >>Mating type switching and transcriptional silencing in Kluyveromyces lactis
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To explore the similarities and differences of regulatory circuits among budding yeasts, we characterized the role of unscheduled meiotic gene expression 6 (UME6) and a novel mating type switching pathway in Kluyveromyces lactis. We found that Ume6 was required for transcriptional silencing of the cryptic mating-type loci HMLα and HMRa. Ume6 acted directly at these loci by binding to the cis-regulatory silencers. Ume6 also served as a block to polyploidy and was required for repression of three meiotic genes, independently of the Rpd3 and Sin3 corepressors.

Mating type switching from MATα to MATa required the α3 protein. The α3 protein was similar to transposases of the mutator like elements (MULEs). Mutational analysis showed that the DDE-motif in α3, which is conserved in MULEs was necessary for switching. During switching α3 mobilizes from the genome in the form of a DNA circle. The sequences encompassing the α3 gene circle junctions in the MATα locus were essential for switching from MATα to MATa. Switching also required a DNA binding protein, Mating type switch 1 (Mts1), whose binding sites in MATα were important. Expression of Mts1 was repressed in MATa/MATα diploids and by nutrients, limiting switching to haploids in low nutrient conditions.

In a genetic selection for strains with increased switching rates we found a mutation in the RAS1 gene. By measuring the levels of the MTS1 mRNA and switching rates in ras1, pde2 and msn2 mutant strains we show that mating type switching in K. lactis was regulated by the RAS/cAMP pathway and the transcription factor Msn2. ras1 mutants contained 20-fold higher levels of MTS1 mRNA compared to wild type whereas pde2 and msn2 expressed less MTS1 mRNA and had decreased switching rates. Furthermore we found that MTS1 contained several potential Msn2 binding sites upstream of its ORF. We suggest that these observations explain the nutrient regulation of switching.

Place, publisher, year, edition, pages
Stockholm: The Wenner-Gren Institute, Stockholm University, 2010. 61 p.
Keyword
Mating type switch, transcriptional silencing, alpha3, transposase, Ume6, Mts1, Ras, cAMP, K. lactis
National Category
Developmental Biology
Research subject
Developmental Biology
Identifiers
urn:nbn:se:su:diva-41969 (URN)978-91-7447-105-2 (ISBN)
Public defence
2010-09-21, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript.Available from: 2010-08-30 Created: 2010-08-12 Last updated: 2010-08-19Bibliographically approved

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