Sex and sex change in fungi

Why is it that some species can change their sexual identity (mating type) during their life time? How do changes of mating type affect the evolutionary process and how does it work? We study these questions in unicellular fungi, where the sexual reproduction (mating) involves fusion of haploid cells of opposite mating types. Several different yeast species can switch their mating type by using cryptic mating type loci and recombination to move mating type cassettes from transcriptionally silent loci into an expressed locus. The process is initiated by highly regulated endonucleases that introduce DNA double strand breaks into DNA. We have determined that the endonucleases that initiate switching have evolved from transposable elements raising questions about the relationship between parasitic DNA and their hosts.

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DNA repair

Before each cell division, cells perform the monumental task of copying (replicating) their entire genome. Replication is extraordinarily accurate as the error-rate (<1 mistake/billion bases) is far lower than any man-made machine. The molecular machine that performs the actual copying is known as the replication fork. If a replication fork encounters a damaged base it may stall and eventually collapse. The cell uses several pathways for restarting replication forks, perhaps the most important one being homologous recombination. During the process of replication fork restart, branched DNA intermediates are formed. The Holliday junction (HJ) is such a branched structure containing a four-way DNA junction, which connects two different chromosomes. HJs are also formed during meiosis probably being the physical bridge that connects homologous chromosomes. We study how HJs are cleaved by nucleases and how this process is regulated.

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DNA double strand breaks, Yeast, mating-type switching, Transposable elements, DNA repair


Selected publications

Chen, J., & Åström, S.U. (2012). A catalytic and non-catalytic role for the Yen1 nuclease in maintaining genome integrity in Kluyveromyces lactis . DNA repair. 11:833-43

Barsoum, E., Rajaei, N., & Åström, S.U. (2011). RAS/cyclic AMP and the transcription factor Msn2 regulate mating and mating-type switching in the yeast Kluyveromyces lactis. Eukaryotic Cell. 10:1545-52

Tsaponina, O., Barsoum, E., Åström, S.U., & Chabes, A. (2011). Ixr1 is required for the expression of the ribonucleotide reductase Rnr1 and maintenance of dNTP pools. PLOS Genetics. 7:e1002061

Barsoum, E., Martinez, P., & Åström. S.U. (2010). Alpha3, a transposable element that promotes host sexual reproduction. Genes Dev. 24:33-44

Carter, S.D., Vigašová, D., Chen, J., Chovanec, M., & Åström, S.U. (2009). Nej1 Recruits the Srs2 Helicase to DNA Double Strand Breaks and Supports Repair by a Single Strand Annealing-like Mechanism. Proc. Natl. Acad. Sci. 106:12037-42