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J. Biol. Chem., Vol. 283, Issue 26, 17766-17776, June 27, 2008

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The Human RecQ Helicases, BLM and RECQ1, Display Distinct DNA Substrate Specificities^*

Venkateswarlu Popuri¹, Csanád Z. Bachrati¹, Laura Muzzolini, Georgina Mosedale, Silvia Costantini, Elisa Giacomini, Ian D. Hickson, and Alessandro Vindigni²

From the International Centre for Genetic Engineering and Biotechnology Padriciano 99, I-34012 Trieste, Italy and the Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DS, United Kingdom

RecQ helicases maintain chromosome stability by resolving a number of highly specific DNA structures that would otherwise impede the correct transmission of genetic information. Previous studies have shown that two human RecQ helicases, BLM and WRN, have very similar substrate specificities and preferentially unwind noncanonical DNA structures, such as synthetic Holliday junctions and G-quadruplex DNA. Here, we extend this analysis of BLM to include new substrates and have compared the substrate specificity of BLM with that of another human RecQ helicase, RECQ1. Our findings show that RECQ1 has a distinct substrate specificity compared with BLM. In particular, RECQ1 cannot unwind G-quadruplexes or RNA-DNA hybrid structures, even in the presence of the single-stranded binding protein, human replication protein A, that stimulates its DNA helicase activity. Moreover, RECQ1 cannot substitute for BLM in the regression of a model replication fork and is very inefficient in displacing plasmid D-loops lacking a 3'-tail. Conversely, RECQ1, but not BLM, is able to resolve immobile Holliday junction structures lacking an homologous core, even in the absence of human replication protein A. Mutagenesis studies show that the N-terminal region (residues 1–56) of RECQ1 is necessary both for protein oligomerization and for this Holliday junction disruption activity. These results suggest that the N-terminal domain or the higher order oligomer formation promoted by the N terminus is essential for the ability of RECQ1 to disrupt Holliday junctions. Collectively, our findings highlight several differences between the substrate specificities of RECQ1 and BLM (and by inference WRN) and suggest that these enzymes play nonoverlapping functions in cells.

Received for publication, November 29, 2007 , and in revised form, March 28, 2008.

^* This work was supported by Cancer Research UK, by grants from the Associazione Italiana per la Ricerca sul Cancro, the Human Frontier Science Program, the Fondo per gli Investimenti della Ricerca di Base of the Ministero dell'Istruzione dell'Università e della Ricerca, and by Consiglio Nazionale delle Ricerche, Rome, Grant 02.00648.ST97. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 39-040-3757369; Fax: 39-040-226555; E-mail: vindigni{at}icgeb.org.

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