Listeria monocytogenes bacterial cells, although harmless when inhabiting soil or water, undergo a marked shift in gene expression that makes them potentially deadly when ingested as food contaminants. Noncoding RNA molecules mediate that shift through "coordinated global transcriptional changes [via] previously unknown regulatory mechanisms," say Pascale Cossart and her colleagues at the Pasteur Institute in Paris. They report their findings in the 17 May 2009 Nature (459:950-956).  

Thus, noncoding RNA molecules, once considered "junk," prove pivotal in regulating the environmental adjustments that these bacteria make when they instigate infections, according to Cossart and her collaborators. "This work is among the first to establish a clear role for noncoding RNAs in bacterial virulence," says Martin Wiedmann, a food microbiologist at Cornell University in Ithaca, N.Y. "It is likely that noncoding RNAs with roles in virulence soon will be identified in a number of other bacterial pathogens. Further understanding of these and other regulatory networks during infection may provide exciting opportunities to develop novel approaches for treatment and prevention of listeriosis and other foodborne diseases."  

Earlier, Cossart and collaborators learned that non-coding RNA molecules determine whether Listeria will infect other cells, and appeared to depend on this ability as a means for survival. Later, when the researchers systematically compared transcription patterns of lab-grown strains of L. monocytogenes, including mutants lacking specific virulence factors, with those of frank pathogens recovered from infected mice and hospital patients, they saw marked differences. "When [Listeria] arrives in the intestine [of mice or humans], it turns up the activity of many genes and turns down others, so we see a dramatic reshaping of the transcriptome program," Cossart says. "Strikingly, a series of noncoding RNAs are expressed more often in the intestine or in the blood."  

In particular, noncoding RNA molecules are linked to the action of two proteins, one called SigB, that controls expression of genes that allow Listeria to adapt to the intestinal environment, and the other called PrfA, that is responsible for turning on genes whose products enable growth in blood. The researchers also identified untranslated regions of RNA that overlap with genes on the opposite DNA strand to regulate their expression.  

More dramatically, these noncoding RNA molecules control expression of upstream genes through about 40 RNA regions called riboswitches, according to Cossart. Previously, riboswitches were recognized as controlling expression of downstream genes in response to amounts of the encoded proteins being made. This upstream control is "completely unexpected," and how they do so remains to be determined, she says. "Noncoding regions have more secrets than previously anticipated."  

"These findings are important as they open up a number of new lines of investigation into the specific contributions of noncoding RNAs and riboswitches in regulation of Listeria monocytogenes gene expression," Wiedmann says. "Understanding involvement of the noncoding RNAs and riboswitches in regulatory networks will identify novel mechanisms used by this food-borne pathogen to adapt to different environments, both in the human host and in external environments, including in foods and food-related environments."  

Brian Hoyle  
Brian Hoyle ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) owns and runs Square Rainbow Ltd., a science writing and editing company.

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