Regulation of telomere addition at DNA double-strand breaks

Telomeres constitute the ends of linear eukaryotic chromosomes. Due to the conventional mode of DNA replication, telomeric DNA erodes at each cell division. To counteract this, a specialized reverse transcriptase, telomerase, can elongate chromosome ends to maintain them at a constant average length. Because of their similarity to DNA double-strand breaks (DSBs), telomeres might be expected to induce a DNA damage response, which would lead to repair reactions and the generation of translocations or fusions. Many proteins present at telomeres prevent this by protecting (capping) the chromosome termini. Conversely, a DSB occurring in other regions of the genome, due, for instance, to a stalled replication fork or genotoxic agents, must be repaired by homologous recombination or end-joining to ensure genome stability. Interestingly, telomerase is able to generate a telomere de novo at an accidental DSB, with potentially lethal consequences in haploid cells and, at a minimum, loss of heterozygosity (LOH) in diploid cells. Recent data suggest that telomerase is systematically recruited to DSBs but is prevented from acting in the absence of a minimal stretch of flanking telomere-repeat sequences. In this review, we will focus on the mechanisms that regulate telomere addition to DSB

Human Pif1 helicase unwinds synthetic DNA structures resembling stalled DNA replication forks

Pif-1 proteins are 5′→3′ superfamily 1 (SF1) helicases that in yeast have roles in the maintenance of mitochondrial and nuclear genome stability. The functions and activities of the human enzyme (hPif1) are unclear, but here we describe its DNA binding and DNA remodeling activities. We demonstrate that hPif1 specifically recognizes and unwinds DNA structures resembling putative stalled replication forks. Notably, the enzyme requires both arms of the replication fork-like structure to initiate efficient unwinding of the putative leading replication strand of such substrates. This DNA structure-specific mode of initiation of unwinding is intrinsic to the conserved core helicase domain (hPifHD) that also possesses a strand annealing activity as has been demonstrated for the RecQ family of helicases. The result of hPif1 helicase action at stalled DNA replication forks would generate free 3′ ends and ssDNA that could potentially be used to assist replication restart in conjunction with its strand annealing activity

The Yeast Pif1 Helicase Prevents Genomic Instability Caused by G-Quadruplex-Forming CEB1 Sequences In Vivo

In budding yeast, the Pif1 DNA helicase is involved in the maintenance of both nuclear and mitochondrial genomes, but its role in these processes is still poorly understood. Here, we provide evidence for a new Pif1 function by demonstrating that its absence promotes genetic instability of alleles of the G-rich human minisatellite CEB1 inserted in the Saccharomyces cerevisiae genome, but not of other tandem repeats. Inactivation of other DNA helicases, including Sgs1, had no effect on CEB1 stability. In vitro, we show that CEB1 repeats formed stable G-quadruplex (G4) secondary structures and the Pif1 protein unwinds these structures more efficiently than regular B-DNA. Finally, synthetic CEB1 arrays in which we mutated the potential G4-forming sequences were no longer destabilized in pif1Δ cells. Hence, we conclude that CEB1 instability in pif1Δ cells depends on the potential to form G-quadruplex structures, suggesting that Pif1 could play a role in the metabolism of G4-forming sequences

Instabilité du minisatellite humain CEB1 chez la levure Saccharomyces cerevisiae en absence de Rad27/FEN1 et Pif1

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Regulation of telomere addition at DNA double-strand breaks

Telomeres constitute the ends of linear eukaryotic chromosomes. Due to the conventional mode of DNA replication, telomeric DNA erodes at each cell division. To counteract this, a specialized reverse transcriptase, telomerase, can elongate chromosome ends to maintain them at a constant average length. Because of their similarity to DNA double-strand breaks (DSBs), telomeres might be expected to induce a DNA damage response, which would lead to repair reactions and the generation of translocations or fusions. Many proteins present at telomeres prevent this by protecting (capping) the chromosome termini. Conversely, a DSB occurring in other regions of the genome, due, for instance, to a stalled replication fork or genotoxic agents, must be repaired by homologous recombination or end-joining to ensure genome stability. Interestingly, telomerase is able to generate a telomere de novo at an accidental DSB, with potentially lethal consequences in haploid cells and, at a minimum, loss of heterozygosity (LOH) in diploid cells. Recent data suggest that telomerase is systematically recruited to DSBs but is prevented from acting in the absence of a minimal stretch of flanking telomere-repeat sequences. In this review, we will focus on the mechanisms that regulate telomere addition to DSBs

Complex Minisatellite Rearrangements Generated in the Total or Partial Absence of Rad27/hFEN1 Activity Occur in a Single Generation and Are Rad51 and Rad52 Dependent

Genomes contain tandem repeat blocks that are at risk of expansion or contraction. The mechanisms of destabilization of the human minisatellite CEB1 (arrays of 36- to 43-bp repeats) were investigated in a previously developed model system, in which CEB1-0.6 (14 repeats) and CEB1-1.8 (42 repeats) alleles were inserted into the genome of Saccharomyces cerevisiae. As in human cells, CEB1 is stable in mitotically growing yeast cells but is frequently rearranged in the absence of the Rad27/hFEN1 protein involved in Okazaki fragments maturation. To gain insight into this mode of destabilization, the CEB1-1.8 and CEB1-0.6 human alleles and 47 rearrangements derived from a CEB1-1.8 progenitor in rad27Δ cells were sequenced. A high degree of polymorphism of CEB1 internal repeats was observed, attesting to a large variety of homology-driven rearrangements. Simple deletion, double deletion, and highly complex events were observed. Pedigree analysis showed that all rearrangements, even the most complex, occurred in a single generation and were inherited equally by mother and daughter cells. Finally, the rearrangement frequency was found to increase with array size, and partial complementation of the rad27Δ mutation by hFEN1 demonstrated that the production of novel CEB1 alleles is Rad52 and Rad51 dependent. Instability can be explained by an accumulation of unresolved flap structures during replication, leading to the formation of recombinogenic lesions and faulty repair, best understood by homology-dependent synthesis-strand displacement and annealing

DNA-end capping by the budding yeast transcription factor and subtelomeric binding protein Tbf1

International audienc

Design and Performance Analysis of Generalized Hopping Codes for FH-CDMA Wireless Systems

跳頻分碼多工系統在無線通訊上的應用日益受到重視，在本論文中，我們提出一種藉著將最大相關性函數值提升到n，使得碼數目得以變多的新式質數碼，稱之為多階層質數碼。當P是一個質數而代表此質數之次方的n是一正整數，我們可得到碼之數目為P的n次方，這些碼之長度為P且最大相關函數值為n。在碼的長度不變，互相關函數性質變為n的情況下，可大幅提高碼的用戶量，並使其在頻寬不變的狀態下，大量增加同時使用系統之用戶的數目。此多階層質數碼可分為n的階層，如任意兩個碼在第l階層，則其相關函式值即為l。我們分析多階層質數碼在開關鍵移跳頻分碼多工系統中之效能。以往只能求得最大互相關函數值為2之碰撞機率，我們提供了最大互相關函數值為n中互相關函數值為n，n - 1，…，1。之碰撞機率可根據不同的需求，在系統效能與資料傳輸速率上選擇較佳的折衷解決辦法。我們的結果顯示在使用者數量與系統效能中，藉著階層的選擇取得有效的平衡。The application of frequency-hopping code-division multiple access (FH-CDMA) to wireless communication systems has been receiving significant attention recently. In this thesis, we study a new family of prime codes, so-called multi-level prime codes, with ex-panded code cardinality by relaxing the maximum cross-correlation function of λc to n, where n is a positive integer. If P is a prime number, our new constructions show that there are Pn sequence of length P and its maximum cross-correlation value is less than or equal to n. These multi-level prime codes can be partitioned into n levels, of which the cross-correlation peak between any two FH codes in level l is at most l ( where l ≤ n ). We analyze the performance of the multi-level prime codes in a conventional ON-OFF key-ing/FH-CDMA (OOK/FH-CDMA) system with hard-limiting detection, and provide the hit probability qn,0, qn,1, …, qn,n of having the cross-correlation values 0, 1, …, n, respectively. Our results show that there is a trade-off between code performance and cardinality.Chapter 1 Introduction 1 1.1 Background 1 1.2 FH-CDMA Systems 3 1.3 Various FH Codes 5 1.3.1 Prime Codes [17] 5 1.3.2 Quadratic Congruence Codes [6] 7 1.3.3 Cubic Codes [7] 10 1.4 Motivation and Outline of Thesis 13 Chapter 2 Construction of Multi-Level Prime Codes 15 2.1 Construction Algorithm 15 2.2 Correlation Property 17 Chapter 3 Performance Analysis of Multi-level Prime Codes 22 3.1 OOK/FH-CDMA Scheme 22 3.2 Effect of Multiple-Access Interference 27 3.3 Effect of Rayleigh Fading 29 3.4 Numerical Results 31 Chapter 4 Conclusions 34 Appendix A 35 Appendix B 36 References 3

FANCD2 Binds MCM Proteins and Controls Replisome Function upon Activation of S Phase Checkpoint Signaling

International audienc