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Journal of Bacteriology, January 2005, p. 276-285, Vol. 187, No. 1
0021-9193/05/$08.00+0     doi:10.1128/JB.187.1.276-285.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Posttranscriptional Repression of GacS/GacA-Controlled Genes by the RNA-Binding Protein RsmE Acting Together with RsmA in the Biocontrol Strain Pseudomonas fluorescens CHA0

Cornelia Reimmann,* Claudio Valverde,{dagger} Elisabeth Kay, and Dieter Haas

Département de Microbiologie Fondamentale, Université de Lausanne, Lausanne, Switzerland

Received 8 April 2004/ Accepted 29 September 2004

In the plant-beneficial soil bacterium Pseudomonas fluorescens CHA0, the production of biocontrol factors (antifungal secondary metabolites and exoenzymes) is controlled at a posttranscriptional level by the GacS/GacA signal transduction pathway involving RNA-binding protein RsmA as a key regulatory element. This protein is assumed to bind to the ribosome-binding site of target mRNAs and to block their translation. RsmA-mediated repression is relieved at the end of exponential growth by two GacS/GacA-controlled regulatory RNAs RsmY and RsmZ, which bind and sequester the RsmA protein. A gene (rsmE) encoding a 64-amino-acid RsmA homolog was identified and characterized in strain CHA0. Overexpression of rsmE strongly reduced the expression of target genes (hcnA, for a hydrogen cyanide synthase subunit; aprA, for the main exoprotease; and phlA, for a component of 2,4-diacetylphloroglucinol biosynthesis). Single null mutations in either rsmA or rsmE resulted in a slight increase in the expression of hcnA, aprA, and phlA. By contrast, an rsmA rsmE double mutation led to strongly increased and advanced expression of these target genes and completely suppressed a gacS mutation. Both the RsmE and RsmA levels increased with increasing cell population densities in strain CHA0; however, the amount of RsmA showed less variability during growth. Expression of rsmE was controlled positively by GacA and negatively by RsmA and RsmE. Mobility shift assays demonstrated specific binding of RsmE to RsmY and RsmZ RNAs. The transcription and stability of both regulatory RNAs were strongly reduced in the rsmA rsmE double mutant. In conclusion, RsmA and RsmE together account for maximal repression in the GacS/GacA cascade of strain CHA0.

* Corresponding author. Mailing address: Département de Microbiologie Fondamentale, Bâtiment de Biologie, Université de Lausanne, CH-1015 Lausanne Dorigny, Switzerland. Phone: 41 21 6925632. Fax: 41 21 6925635. E-mail: Cornelia.Reimmann{at}unil.ch.

{dagger} Present address: PIIB, Departamento de Ciencia y Tecnología, Universidad Nacional de Quilmes, Bernal, Argentina.

Journal of Bacteriology, January 2005, p. 276-285, Vol. 187, No. 1
0021-9193/05/$08.00+0     doi:10.1128/JB.187.1.276-285.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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