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Journal of Bacteriology, February 2002, p. 1046-1056, Vol. 184, No. 4
0021-9193/01/$04.00+0     DOI: 10.1128/jb.184.4.1046-1056.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Regulatory RNA as Mediator in GacA/RsmA-Dependent Global Control of Exoproduct Formation in Pseudomonas fluorescens CHA0

Stephan Heeb, Caroline Blumer,{dagger}, and Dieter Haas*

Laboratoire de Biologie Microbienne, Université de Lausanne, CH-1015 Lausanne, Switzerland

Received 16 August 2001/ Accepted 19 November 2001

In Pseudomonas fluorescens CHA0, an antagonist of root-pathogenic fungi, the GacS/GacA two-component system tightly controls the expression of antifungal secondary metabolites and exoenzymes at a posttranscriptional level, involving the RNA-binding protein and global regulator of secondary metabolism RsmA. This protein was purified from P. fluorescens, and RNA bound to it was converted to cDNA, which served as a probe to isolate the corresponding chromosomal locus, rsmZ. This gene encoded a regulatory RNA of 127 nucleotides and a truncated form lacking 35 nucleotides at the 3" end. Expression of rsmZ depended on GacA, increased with increasing population density, and was stimulated by the addition of a solvent-extractable extracellular signal produced by strain CHA0 at the end of exponential growth. This signal appeared to be unrelated to N-acyl-homoserine lactones. A conserved upstream element in the rsmZ promoter, but not the stress sigma factor RpoS, was involved in rsmZ expression. Overexpression of rsmZ effectively suppressed the negative effect of gacS and gacA mutations on target genes, i.e., hcnA (for hydrogen cyanide synthase) and aprA (for the major exoprotease). Mutational inactivation of rsmZ resulted in reduced expression of these target genes in the presence of added signal. Overexpression of rsmA had a similar, albeit stronger negative effect. These results support a model in which GacA upregulates the expression of regulatory RNAs, such as RsmZ of strain CHA0, in response to a bacterial signal. By a titration effect, RsmZ may then alleviate the repressing activity of RsmA on the expression of target mRNAs.

* Corresponding author. Mailing address: Laboratoire de Biologie Microbienne, Université de Lausanne, CH-1015 Lausanne, Switzerland. Phone: 41 21 692 56 31. Fax: 41 21 692 56 35. E-mail: Dieter.Haas{at}lbm.unil.ch.

{dagger} Present address: Functional Genomics, Aventis Pharma Deutschland GmbH, D-65926 Frankfurt am Main, Germany.

Journal of Bacteriology, February 2002, p. 1046-1056, Vol. 184, No. 4
0021-9193/01/$04.00+0     DOI: 10.1128/jb.184.4.1046-1056.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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