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Journal of Bacteriology, July 2009, p. 4180-4185, Vol. 191, No. 13
0021-9193/09/$08.00+0     doi:10.1128/JB.01707-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Active Transcription of rRNA Operons Condenses the Nucleoid in Escherichia coli: Examining the Effect of Transcription on Nucleoid Structure in the Absence of Transertion{triangledown}

Julio E. Cabrera,1,{dagger},{ddagger} Cedric Cagliero,1,{dagger} Selwyn Quan,2 Catherine L. Squires,2 and Ding Jun Jin1*

Transcription Control Section, Gene Regulation and Chromosome Biology Laboratory, National Cancer Institute-Frederick, National Institutes of Health, Frederick, Maryland 21702,1 Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 021112

Received 6 December 2008/ Accepted 5 April 2009

In Escherichia coli the genome must be compacted ~1,000-fold to be contained in a cellular structure termed the nucleoid. It is proposed that the structure of the nucleoid is determined by a balance of multiple compaction forces and one major expansion force. The latter is mediated by transertion, a coupling of transcription, translation, and translocation of nascent membrane proteins and/or exported proteins. In supporting this notion, it has been shown consistently that inhibition of transertion by the translation inhibitor chloramphenicol results in nucleoid condensation due to the compaction forces that remain active in the cell. Our previous study showed that during optimal growth, RNA polymerase is concentrated into transcription foci or "factories," analogous to the eukaryotic nucleolus, indicating that transcription and RNA polymerase distribution affect the nucleoid structure. However, the interpretation of the role of transcription in the structure of the nucleoid is complicated by the fact that transcription is implicated in both compacting forces and the expansion force. In this work, we used a new approach to further examine the effect of transcription, specifically from rRNA operons, on the structure of the nucleoid, when the major expansion force was eliminated. Our results showed that transcription is necessary for the chloramphenicol-induced nucleoid compaction. Further, an active transcription from multiple rRNA operons in chromosome is critical for the compaction of nucleoid induced by inhibition of translation. All together, our data demonstrated that transcription of rRNA operons is a key mechanism affecting genome compaction and nucleoid structure.

* Corresponding author. Mailing address: Gene Regulation and Chromosome Biology Laboratory, National Cancer Institute-Frederick, National Institutes of Health, 1050 Boyles St., Frederick, MD 21702. Phone: (301) 846-7684. Fax: (301) 846-1489. E-mail: djjin{at}helix.nih.gov

{triangledown} Published ahead of print on 24 April 2009.

{dagger} These two authors contributed equally to this work.

{ddagger} Present address: ProteinOne, Inc., Bethesda, MD 20814.

Journal of Bacteriology, July 2009, p. 4180-4185, Vol. 191, No. 13
0021-9193/09/$08.00+0     doi:10.1128/JB.01707-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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