How small RNAs affect Salmonella infection

Salmonella is a foodborne pathogen
that infects millions of people each year.
Infection by these bacteria relies on a complex network of genes and gene products that enable them to sense environmental conditions
.
In a new paper, researchers at the University of Illinois at Urbana-Champaign study how small RNAs help salmonella express its virulence genes
.

When infecting humans, Salmonella first invades intestinal cells
through a needle-like structure called the type III secretion system.
This structure injects proteins directly into cells, triggering a cascade of changes that cause inflammation and eventually diarrhea
.
The genes encoding this system, along with other genes needed for invasion, are located in a DNA region
called Salmonella Pathogenic Island 1 (SPI-1).

Sabrina Abdulla, lead author of the study, said: "SPI-1 needs to be well controlled
.
If a type III secretory system cannot be produced, Salmonella will not cause infection, but if the needle-like structure is produced too much, Salmonella will get sick
.
”

As a result, SPI-1 is subject to strict regulatory network controls
.
First, three transcription factors, HilD, HilC, and RtsA, control their own and each other's DNA expression
.
They also activated another transcription factor, HilA, which activates the rest of the SPI-1 gene
.
If that's not complicated enough, SPI-1 also needs to sense environmental cues and adjust its gene expression in order to infect the host
.

"We have known for a long time that there are many environmental factors that influence the gene regulation
of Salmonella.
However, we don't know how to do that
.
Since then, researchers have focused on small RNA," Abdulla said
.

Small RNAs (sRNA) determine how genes function
in bacterial cells.
Typically, these molecules either interact with proteins or mRNA
.
As a result, sRNA affects a variety of bacterial functions, including virulence and response to
the environment.

In this study, they analyzed in depth the sRNA that regulates hilD mRNA, specifically a sequence on the mRNA 3' untranslated region, which is not involved in the production of
the HilD protein.
In bacteria, 3' UTRs typically contain 50-100 nucleotides
.
However, the 3' UTR of hilD mRNA is up to 300 nucleotides
long.

"We observed a 60-fold increase in the expression of the hilD gene after deletion of the 3' UTR," Abdulla said
.
"So we decided to look for sRNA
that might interact with the region.
"

The researchers found that although both sRNAs, Spot 42 and SdsR, can target 3' UTR, their regions of action differ
.
"This result suggests that the entire 3'UTR is important for regulation," Abdulla said
.
"We found that sRNA stabilizes hilD mRNA and protects it from degradation
.
"

The researchers also used mice to see if Spot 42 and SdsR affected Salmonella infection
.
They conducted mouse competition experiments, introducing mutant bacteria lacking sRNA and normal bacteria containing sRNA to see which strains survived and caused infection
.
"We found that when the sRNA is deleted, the bacteria cannot survive
in the host.
We also showed that sRNA plays a role in helping SPI-1 invade host cells," Abdulla said
.

"Now we know that sRNA plays an important role in controlling SPI-1, but we also want to expand our research
in two directions.
" We wanted to understand how sRNA affects the mRNA level of hilD at the molecular level
.
We also want to better understand how sRNA regulates the expression of other important SPI-1 genes," said
corresponding author Cari Vanderpool, professor of microbiology.

Original search

Small RNAs Activate Salmonella Pathogenicity Island 1 by Modulating mRNA Stability through the hilD mRNA 3′ Untranslated Region