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Friday Science Review: December 3, 2010

It’s all about microscopic machinery this week with two articles in Molecular Cell (Cell Press) and a third in Nature Cell Biology.

The MMS22L-TONSL Complex to the Rescue: A Sine Qua Non for Genome Integrity

Samuel Lunenfeld Research Institute ♦ University of Toronto

Published in Molecular Cell, November 24, 2010

In order for DNA replication to occur smoothly, a large complex of DNA polymerases and other proteins must work in harmony and navigate their way down the length of double-stranded DNA to synthesize daughter strands. This machinery, known as the ‘replisome’, frequently stumbles upon genomic glitches and other impediments that have the potential to hinder its progress. As a result, the cell has evolved a basket of mechanisms to ensure that the replisome avoids stalling and the replication fork continues to move. In a study led by Dr. Anne-Claude Gingras of the Samuel Lunenfeld Research Insitute, scientists use an RNAi screen to identify MMS22L-TONSL — a complex that appears to rescue the replisome during times of replicative stress. The newly identified complex exerts its rescue effects by interacting with single-stranded DNA (ssDNA) during end processing or in regions where the replication fork has stalled. After seeking out ssDNA MMS22L-TONSL goes to work catalyzing the repair of faulty DNA lesions, opening a path forward for the replisome.

MicroRNA Families Modulate Embryonic Messenger Transcripts

McGill University

Published in Molecular Cell, November 24, 2010

It appears microRNA (miRNA) controls expression of messenger RNA targets at the embryonic stage. These special RNA molecules originate in the nucleus, much like mRNA, but are subsequently modified by the enzyme RNaseIII and then exported to the cytoplasm. It is here that they are cleaved and manipulated into their mature form by the enzyme Dicer. After being processed miRNAs are incorporated into silencing complexes that then sort through the mRNA content of the cytoplasm, silencing specific transcripts as they go.  Dr. Thomas Duchaine and his colleagues utilized a C. elegans model to show that two embryonic miRNA families contribute to a natural RNAi process by suppressing expression of target mRNAs. The group also shows that silencing, achieved through epigenetic modification of targets, occurs in a target-specific manner with a unique modification pattern provided to each mRNA target.

Molecular Maintenance of Centromeres, GTPase Pulls the Switch

University of Montreal

Published in Nature Cell Biology, November 21, 2010

The central region of the chromosome has the responsibility of controlling chromosomal separation during cellular division. Like almost all parts of the genome, this region, known as the centromere, is subject to epigenetic regulation. The specialized H3 histone CENP-A is found exclusively at centromeres and is believed to be the epigenetic label of the region. Dr. Paul Maddox and his team at the University of Montreal have recently discovered new agents that maintain the assembly of CENP-A following its addition to the centromere region. A GTPase activating protein interacts with a CENP-A factor to recruit a number of auxiliary proteins that play an essential role in stabilizing newly added CENP-A. This stabilization process early on in the cell cycle is critical in ensuring that each new chromosome receives a sufficient quantity of CENP-A following cell division.

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