The dual role of autonomously replicating sequences as origins of replication and as silencers.
ABSTRACT Autonomously replicating sequences (ARSs) in Saccharomyces cerevisiae have been extensively characterized as both origins of DNA replication and as chromatin repressors/silencers. It has been conclusively shown that the origin and the silencer activities of ARS are substantially, but not entirely interchangeable and that they are modulated by position effects and chromatin environment. It remains unclear how these two quite divergent functions of ARS co-exist. This perspective focuses on recent advances, which have shown that slight differences in ARSs can modulate their affinity for origin recognition complex and their activity as silencers or origins.
Article: Modulation of telomeres in alternative lengthening of telomeres type I like human cells by the expression of werner protein and telomerase.[show abstract] [hide abstract]
ABSTRACT: The alternative lengthening of telomeres (ALT) is a recombination-based mechanism of telomere maintenance activated in 5-20% of human cancers. In Saccharomyces cerevisiae, survivors that arise after inactivation of telomerase can be classified as type I or type II ALT. In type I, telomeres have a tandem array structure, with each subunit consisting of a subtelomeric Y' element and short telomere sequence. Telomeres in type II have only long telomere repeats and require Sgs1, the S. cerevisiae RecQ family helicase. We previously described the first human ALT cell line, AG11395, that has a telomere structure similar to type I ALT yeast cells. This cell line lacks the activity of the Werner syndrome protein, a human RecQ helicase. The telomeres in this cell line consist of tandem repeats containing SV40 DNA, including the origin of replication, and telomere sequence. We investigated the role of the SV40 origin of replication and the effects of Werner protein and telomerase on telomere structure and maintenance in AG11395 cells. We report that the expression of Werner protein facilitates the transition in human cells of ALT type I like telomeres to type II like telomeres in some aspects. These findings have implications for the diagnosis and treatment of cancer.Journal of Oncology 01/2012; 2012:806382.
Article: Design of a minimal silencer for the silent mating-type locus HML of Saccharomyces cerevisiae.[show abstract] [hide abstract]
ABSTRACT: The silent mating-type loci HML and HMR of Saccharomyces cerevisiae contain mating-type information that is permanently repressed. This silencing is mediated by flanking sequence elements, the E- and I-silencers. They contain combinations of binding sites for the proteins Rap1, Abf1 and Sum1 as well as for the origin recognition complex (ORC). Together, they recruit other silencing factors, foremost the repressive Sir2/Sir3/Sir4 complex, to establish heterochromatin-like structures at the HM loci. However, the HM silencers exhibit considerable functional redundancy, which has hampered the identification of further silencing factors. In this study, we constructed a synthetic HML-E silencer (HML-SS ΔI) that lacked this redundancy. It consisted solely of Rap1 and ORC-binding sites and the D2 element, a Sum1-binding site. All three elements were crucial for minimal HML silencing, and mutations in these elements led to a loss of Sir3 recruitment. The silencer was sensitive to a mutation in RAP1, rap1-12, but less sensitive to orc mutations or sum1Δ. Moreover, deletions of SIR1 and DOT1 lead to complete derepression of the HML-SS ΔI silencer. This fully functional, minimal HML-E silencer will therefore be useful to identify novel factors involved in HML silencing.Nucleic Acids Research 12/2010; 38(22):7991-8000. · 8.03 Impact Factor
Article: Sub-telomeric core X and Y' elements in S. cerevisiae suppress extreme variations in gene silencing.[show abstract] [hide abstract]
ABSTRACT: Telomere Position Effect (TPE) is governed by strong repression signals emitted by telomeres via the Sir2/3/4 Histone Deacetylase complex. These signals are then relayed by weak proto-silencers residing in the subtelomeric core X and Y' elements. Subtelomeres also contain Sub-Telomeric Anti-silencing Regions (STARs). In this study we have prepared telomeres built of different combinations of core X, Y' and STARs and have analyzed them in strains lacking Histone-Acetyltransferase genes as well as in cdc6-1 and Δrif1 strains. We show that core X and Y' dramatically reduce both positive and negative variations in TPE, that are caused by these mutations. We also show that the deletion of Histone-Acetyltransferase genes reduce the silencing activity of an ACS proto-silencer, but also reduce the anti-silencing activity of a STAR. We postulate that core X and Y' act as epigenetic "cushioning" cis-elements.PLoS ONE 01/2011; 6(3):e17523. · 4.09 Impact Factor