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Eukaryotic Cell, August 2002, p. 613-625, Vol. 1, No. 4
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.4.613-625.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

The Fission Yeast Ubiquitin-Conjugating Enzymes UbcP3, Ubc15, and Rhp6 Affect Transcriptional Silencing of the Mating-Type Region

Inga Sig Nielsen,1 Olaf Nielsen,1 Johanne M. Murray,2 and Geneviève Thon1*

Department of Genetics, Institute of Molecular Biology, University of Copenhagen, DK-1353 Copenhagen K, Denmark,1 School of Biological Sciences, Sussex University, Falmer, Brighton BN1 9QG, United Kingdom2

Received 24 September 2001/ Accepted 4 June 2002

Genes transcribed by RNA polymerase II are silenced when introduced near the mat2 or mat3 mating-type loci of the fission yeast Schizosaccharomyces pombe. Silencing is mediated by a number of gene products and cis-acting elements. We report here the finding of novel trans-acting factors identified in a screen for high-copy-number disruptors of silencing. Expression of cDNAs encoding the putative E2 ubiquitin-conjugating enzymes UbcP3, Ubc15 (ubiquitin-conjugating enzyme), or Rhp6 (Rad homolog pombe) from the strong nmt1 promoter derepressed the silent mating-type loci mat2 and mat3 and reporter genes inserted nearby. Deletion of rhp6 slightly derepressed an ade6 reporter gene placed in the mating-type region, whereas disruption of ubcP3 or ubc15 had no obvious effect on silencing. Rhp18 is the S. pombe homolog of Saccharomyces cerevisiae Rad18p, a DNA-binding protein that physically interacts with Rad6p. Rhp18 was not required for the derepression observed when UbcP3, Ubc15, or Rhp6 was overproduced. Overexpressing Rhp6 active-site mutants showed that the ubiquitin-conjugating activity of Rhp6 is essential for disruption of silencing. However, high dosage of UbcP3, Ubc15, or Rhp6 was not suppressed by a mutation in the 26S proteasome, suggesting that loss of silencing is not due to an increased degradation of silencing factors but rather to the posttranslational modification of proteins by ubiquitination. We discuss the implications of these results for the possible modes of action of UbcP3, Ubc15, and Rhp6.

* Corresponding author. Mailing address: Department of Genetics, Institute of Molecular Biology, University of Copenhagen, Øster Farimagsgade 2 A, DK-1353 Copenhagen K, Denmark. Phone: 45 35 32 21 08. Fax: 45 35 32 21 13. E-mail: gen{at}biobase.dk.

Eukaryotic Cell, August 2002, p. 613-625, Vol. 1, No. 4
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.4.613-625.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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