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Journal of Bacteriology, November 2008, p. 7346-7356, Vol. 190, No. 22
0021-9193/08/$08.00+0     doi:10.1128/JB.00680-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Hypernegative Supercoiling Inhibits Growth by Causing RNA Degradation{triangledown}

Imad Baaklini,1 Valentine Usongo,1 Flora Nolent,1 Patrick Sanscartier,1 Chadi Hraiky,1 Karl Drlica,2 and Marc Drolet1*

Département de Microbiologie et Immunologie, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, Québec H3C 3J7, Canada,1 Public Health Research Institute, New Jersey Medical School, Newark, New Jersey 071032

Received 14 May 2008/ Accepted 1 September 2008

Transcription-induced hypernegative supercoiling is a hallmark of Escherichia coli topoisomerase I (topA) mutants. However, its physiological significance has remained unclear. Temperature downshift of a mutant yielded transient growth arrest and a parallel increase in hypernegative supercoiling that was more severe with lower temperature. Both properties were alleviated by overexpression of RNase HI. While ribosomes in extracts showed normal activity when obtained during growth arrest, mRNA on ribosomes was reduced for fis and shorter for crp, polysomes were much less abundant relative to monosomes, and protein synthesis rate dropped, as did the ratio of large to small proteins. Altered processing and degradation of lacA and fis mRNA was also observed. These data are consistent with truncation of mRNA during growth arrest. These effects were not affected by a mutation in the gene encoding RNase E, indicating that this endonuclease is not involved in the abnormal mRNA processing. They were also unaffected by spectinomycin, an inhibitor of protein synthesis, which argued against induction of RNase activity. In vitro transcription revealed that R-loop formation is more extensive on hypernegatively supercoiled templates. These results allow us, for the first time, to present a model by which hypernegative supercoiling inhibits growth. In this model, the introduction of hypernegative supercoiling by gyrase facilitates degradation of nascent RNA; overproduction of RNase HI limits the accumulation of hypernegative supercoiling, thereby preventing extensive RNA degradation.

* Corresponding author. Mailing address: Département de Microbiologie et Immunologie, Université de Montréal, 2900 Édouard-Montpetit, Room R-612, Montréal, Québec H3C 3J7, Canada. Phone: (514) 343-5796. Fax: (514) 343-5701. E-mail: marc.drolet{at}umontreal.ca

{triangledown} Published ahead of print on 12 September 2008.

Journal of Bacteriology, November 2008, p. 7346-7356, Vol. 190, No. 22
0021-9193/08/$08.00+0     doi:10.1128/JB.00680-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Gomez-Gonzalez, B., Aguilera, A. (2009). R-loops do not accumulate in transcription-defective hpr1-101 mutants: implications for the functional role of THO/TREX. Nucleic Acids Res 37: 4315-4321 [Abstract] [Full Text]  
  • Liu, Z., Deibler, R. W., Chan, H. S., Zechiedrich, L. (2009). The why and how of DNA unlinking. Nucleic Acids Res 37: 661-671 [Abstract] [Full Text]  


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