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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2008.08.037 Byline: Zhu Zhu (1), Woo-Hyun Chung (1), Eun Yong Shim (2), Sang Eun Lee (2), Grzegorz Ira (1) Keywords: DNA Abstract: Formation of single-strand DNA (ssDNA) tails at a double-strand break (DSB) is a key step in homologous recombination and DNA-damage signaling. The enzyme(s) producing ssDNA at DSBs in eukaryotes remain unknown. We monitored 5'-strand resection at inducible DSB ends in yeast and identified proteins required for two stages of resection: initiation and long-range 5'-strand resection. We show that the Mre11-Rad50-Xrs2 complex (MRX) initiates 5' degradation, whereas Sgs1 and Dna2 degrade 5' strands exposing long 3' strands. Deletion of SGS1 or DNA2 reduces resection and DSB repair by single-strand annealing between distant repeats while the remaining long-range resection activity depends on the exonuclease Exo1. In exo1[DELTA]sgs1[DELTA] double mutants, the MRX complex together with Sae2 nuclease generate, in a stepwise manner, only few hundred nucleotides of ssDNA at the break, resulting in inefficient gene conversion and G2/M damage checkpoint arrest. These results provide important insights into the early steps of DSB repair in eukaryotes. Author Affiliation: (1) Department of Molecular & Human Genetics, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA (2) Department of Molecular Medicine and Institute of Biotechnology, University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, San Antonio, TX 78245, USA Article History: Received 15 May 2008; Revised 30 July 2008; Accepted 27 August 2008 Article Note: (miscellaneous) Published: September 18, 2008

Research has been conducted on metazoan transcription elongation factor P-TEFb (CDK-9/cyclin T). The study of the P-TEFb functions in vivo has been carried out via the use of RNA interference in the Caenorhabditis elegans embryo and the results are presented.

Transcriptional repression by a Caenorhabditis elegans germ-line protein has been studied. In the early C. elegans embryo, PIE-1 protein, which is maternally expressed, is necessary in germ-line blastomeres to inhibit somatic differentiation, block phosphorylation of the RNA polymerase II large subunit (Pol II) carboxy-terminal domain (CTD) and maintain mRNA transcription absence. It has been found the PIE-1 can act as a transcriptional repressor in cell culture assays. A PIE-1 repression domain that seems to cut back transcriptional machinery directly has been identified. It seems repressor activity may be important for in vivo PIE-1 function.

Nucleosome positioning is still under investigation. The mice transcriptional enhancer of the serum albumin gene acts in the liver. There regulatory factors occupy target sites on three particles designated N1, N2 and N3. They are nucleosome-like. Winged helix transcription factor HNF3 binds to two near-center sites in the N1 particle. Dinucleosome templates using the albumin enhancer sequence were created. Site-specific binding of HNF3 protein brought nucleosome positioning in vitro like that seen in liver nuclei. Binding of a transcription factor can put in place an underlying nucleosome core.