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J Biol Chem, Vol. 275, Issue 15, 11257-11263, April 14, 2000

RNase H Overproduction Corrects a Defect at the Level of Transcription Elongation during rRNA Synthesis in the Absence of DNA Topoisomerase I in Escherichia coli^*

Chadi Hraiky, Marc-André Raymond, and Marc Drolet

From the Département de Microbiologie et Immunologie, Université de Montréal, Québec H3C 3J7, Canada

It has been suggested that the major function of DNA topoisomerase I in Escherichia coli is to suppress the formation of R-loops, which could inhibit growth. Although the currently available data suggest that the inhibitory effect of R-loops is exerted at the level of gene expression, this has never been demonstrated. In the present report, we show that rRNA synthesis is significantly impaired at the level of transcription elongation in a bacterial strain lacking DNA topoisomerase I. We found that this inhibition is due to transcriptional blocks. RNase H overproduction is also shown to considerably reduce the extent of such transcriptional blocks during rRNA synthesis. Moreover, one of these transcriptional blockage sites is located within a region where extensive R-loop formation was previously shown to occur on a plasmid DNA in the absence of DNA topoisomerase I. Together, these results allow us to propose that an important function of DNA topoisomerase I is to inhibit the formation of R-loops, which may otherwise translate into roadblocks for RNA polymerases. Our results also highlight the potential regulatory role of DNA supercoiling at the level of transcription elongation.

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^* This work was supported by Grant MT-12667 from the Medical Research Council of Canada (to M. D.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Recipient of a scholarship from Les Fonds de la Recherche en Santé du Québec. To whom correspondence should be addressed: Département de Microbiologie et Immunologie, Université de Montréal, C. P. 6128, Succursale Centre-ville, Montréal, Québec H3C 3J7, Canada. Tel.: 514-343-5796; Fax: 514-343-5701; E-mail: Marc.Drolet@umontreal.ca.

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Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.

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