Effects of perinuclear chromosome tethers in the telomeric URA3/5FOA system reflect changes to gene silencing and not nucleotide metabolism
- 1 Faculty of Medicine, Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON, Canada
- 2 Canada Research Chairs Program, Faculty of Medicine, University of Toronto, Toronto, ON, Canada
Telomeres are repetitive DNA sequences that protect the ends of linear chromosomes. Telomeres also recruit histone deacetylase complexes that can then spread along chromosome arms and repress the expression of subtelomeric genes in a process known as telomere position effect (TPE). In the budding yeast Saccharomyces cerevisiae, association of telomeres with the nuclear envelope is thought to promote TPE by increasing the local concentration of histone deacetylase complexes at chromosome ends. Importantly, our understanding of TPE stems primarily from studies that employed marker genes inserted within yeast subtelomeres. In particular, the prototrophic marker URA3 is commonly used to assay TPE by negative selection on media supplemented with 5-fluoro-orotic acid (5FOA). Recent findings suggested that decreased growth on 5FOA-containing media may not always indicate increased expression of a telomeric URA3 reporter, but can rather reflect an increase in ribonucleotide reductase (RNR) function and nucleotide metabolism. Thus, we set out to test if the 5FOA sensitivity of subtelomeric URA3-harboring cells in which we deleted various factors implicated in perinuclear telomere tethering reflects changes to TPE and/or RNR. We report that RNR inhibition restores 5FOA resistance to cells lacking RNR regulatory factors but not any of the major telomere tethering and silencing factors, including Sir2, cohibin, Mps3, Heh1, and Esc1. In addition, we find that the disruption of tethering pathways in which these factors participate increases the level of URA3 transcripts originating from the telomeric reporter gene and abrogates silencing of subtelomeric HIS3 reporter genes without altering RNR gene expression. Thus, increased 5FOA sensitivity of telomeric URA3-harboring cells deficient in telomere tethers reflects the dysregulation of TPE but not RNR. This is key to understanding relationships between telomere positioning, chromatin silencing, and lifespan.
Keywords: SIR, cohibin, Esc1, Mps3, Heh1, telomere position effect, ribonucleotide reductase, URA3/5FOA
Citation: Poon BPK and Mekhail K (2012) Effects of perinuclear chromosome tethers in the telomeric URA3/5FOA system reflect changes to gene silencing and not nucleotide metabolism. Front. Gene. 3:144. doi: 10.3389/fgene.2012.00144
Received: 10 April 2012; Accepted: 16 July 2012;
Published online: 02 August 2012.
, Perelman School of Medicine at the University of Pennsylvania, USA
Copyright: © 2012 Poon and Mekhail. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: Karim Mekhail, Medical Sciences Building, Room 6342, 1 King’s College Circle, University of Toronto, Toronto, ON, Canada M5S 1A8. e-mail: email@example.com