Regulatory RNA as Mediator in GacA/RsmA-Dependent Global Control of Exoproduct Formation inPseudomonas fluorescensCHA0

Abstract

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.

The LetE Protein Enhances Expression of Multiple LetA/LetS-Dependent Transmission Traits by Legionella pneumophila

“…The results of the Northern blot analysis indicated that L. pneumophila expresses an ϳ500-base monocistronic letE RNA in the E phase; in the PE phase, LetA/LetS and RpoS regulate its decay. Its scarcity in the PE phase is inconsistent with the idea that letE encodes a regulatory RNA that sequesters CsrA to relieve repression of the transmission phenotype (20).…”

“…Members of this family of two-component regulators induce the expression of the csrB regulatory RNA, which sequesters the CsrA repressor, permitting the translation of target mRNAs (20). However, unlike csrB, L. pneumophila LetE appears to function as a protein, not an RNA.…”

“…First, in a variety of other gram-negative bacteria, homologues of LetA/LetS activate gene expression indirectly by counteracting the effects of CsrA, a repressor of translation (8,12,20,21). Secondly, in PE-phase L. pneumophila, LetA/LetS and RpoS may mediate the decay of letE RNA (Fig.…”

“…CsrA is antagonized by an untranslated regulatory RNA, known as csrB or rsmB, which contains multiple repeats of a ribosome binding site-like sequence that presumably bind CsrA and sequester it from mRNAs (32). LetA/LetS orthologues activate csrB expression, thereby promoting the translation of transcripts bound by CsrA (1,20). For example, the overproduction of csrB by Salmonella enterica serovar Typhimurium bypasses its requirement for the LetA orthologue BarA, as judged by its more efficient invasion of cultured epithelial cells (2).…”

Differential regulation of phenazine biosynthesis by RpeA and RpeB in Pseudomonas chlororaphis 30-84

“…The best known TCST among Pseudomonas species is GacS/GacA, known as a master regulator of secondary metabolism. GacS/GacA is essential for phenazine production in P. chlororaphis (Chancey et al, 1999(Chancey et al, , 2002 and other pseudomonads (Heeb et al, 2002). Upon activation by as-yet undetermined signals, the membrane-bound HK sensor GacS activates the GacA RR by phosphorylation.…”