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Journal of Bacteriology, March 2009, p. 1738-1748, Vol. 191, No. 6
0021-9193/09/$08.00+0     doi:10.1128/JB.01524-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Autogenous Regulation of Escherichia coli Polynucleotide Phosphorylase Expression Revisited ^,

Thomas Carzaniga,¹ Federica Briani,¹ Sandro Zangrossi,² Giuseppe Merlino,¹ Paolo Marchi,^1,3, and Gianni Dehò^1*

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Milan, Italy,¹ Centro di Studio del CNR sulla Biologia Cellulare e Molecolare delle Piante, Milan, Italy,² Laboratorio di Genetica, Dipartimento di Biologia, Università di Camerino, Camerino, MC, Italy³

Received 28 October 2008/ Accepted 4 January 2009

The Escherichia coli polynucleotide phosphorylase (PNPase; encoded by pnp), a phosphorolytic exoribonuclease, posttranscriptionally regulates its own expression at the level of mRNA stability and translation. Its primary transcript is very efficiently processed by RNase III, an endonuclease that makes a staggered double-strand cleavage about in the middle of a long stem-loop in the 5'-untranslated region. The processed pnp mRNA is then rapidly degraded in a PNPase-dependent manner. Two non-mutually exclusive models have been proposed to explain PNPase autogenous regulation. The earlier one suggested that PNPase impedes translation of the RNase III-processed pnp mRNA, thus exposing the transcript to degradative pathways. More recently, this has been replaced by the current model, which maintains that PNPase would simply degrade the promoter proximal small RNA generated by the RNase III endonucleolytic cleavage, thus destroying the double-stranded structure at the 5' end that otherwise stabilizes the pnp mRNA. In our opinion, however, the first model was not completely ruled out. Moreover, the RNA decay pathway acting upon the pnp mRNA after disruption of the 5' double-stranded structure remained to be determined. Here we provide additional support to the current model and show that the RNase III-processed pnp mRNA devoid of the double-stranded structure at its 5' end is not translatable and is degraded by RNase E in a PNPase-independent manner. Thus, the role of PNPase in autoregulation is simply to remove, in concert with RNase III, the 5' fragment of the cleaved structure that both allows translation and prevents the RNase E-mediated PNPase-independent degradation of the pnp transcript.

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* Corresponding author. Mailing address: Dipartimento di Scienze biomolecolari e Biotecnologie, Università degli Studi di Milano, Via Celoria 26 / A4, 20133 Milano. Phone: (39)02.5031.5019. Fax: (39)02.5031.5044. E-mail: gianni.deho{at}unimi.it

Published ahead of print on 9 January 2009.

Supplemental material for this article may be found at http://jb.asm.org/.

Present address: Department of Pharmacology, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.

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Journal of Bacteriology, March 2009, p. 1738-1748, Vol. 191, No. 6
0021-9193/09/$08.00+0     doi:10.1128/JB.01524-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.