Abstract
ABSTRACT The facultative intracellular pathogen Listeria monocytogenes can persist and grow in a diverse range of environmental conditions, both outside and within its mammalian host. The alternative sigma factor Sigma B (σB) plays an important role in this adaptability and is critical for the transition into the host. While some of the functions of the σB regulon in facilitating this transition are understood the role of σB-dependent small regulatory RNAs (sRNAs) remain poorly characterized. In this study, we focused on elucidating the function of Rli47, a σB-dependent sRNA that is highly induced in the intestine and in macrophages. Using a combination of in silico and in vivo approaches, a binding interaction was predicted with the Shine-Dalgarno region of the ilvA mRNA, which encodes threonine deaminase, an enzyme required for branched-chain amino acid biosynthesis. Both ilvA transcript levels and threonine deaminase activity were increased in a deletion mutant lacking the rli47 gene. The Δrli47 mutant displayed a shorter growth lag in isoleucine-depleted growth media relative to the wild-type, and a similar phenotype was also observed in a mutant lacking σB. The impact of the Δrli47 on the global transcription profile of the cell was investigated using RNA-seq, and a significant role for Rli47 in modulating amino acid metabolism was uncovered. Taken together, the data point to a model where Rli47 is responsible for specifically repressing isoleucine biosynthesis as a way to restrict growth under harsh conditions, potentially contributing to the survival of L. monocytogenes in niches both outside and within the mammalian host.
Highlights
The Gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen widely found in the environment [1]
Nucleotide BLAST searches [24] against other Listeria species for which whole genome sequence is available revealed that rli47 is present in all Listeria sensu stricto species (L. monocytogenes, L. innocua, L. seeligeri, L. ivanovii, L. welshimeri and L. marthii) but was not found in the genomes of species of the Listeria sensu lato group (L. weihenstephanensis, L. fleschmannii, L. floridensis, L. aquatica, L. newyorkensis, L. cornellensis, L. grandensis, L. riparia, L. booriae, L. rocourtiae)
There may be a negative effect of Rli47 on ilvA mRNA levels since there was a significant increase in the ilvA transcript in the Δrli47 mutant strain cells grown in TSB (Fig. S6)
Summary
The Gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen widely found in the environment [1]. While transitioning from a saprophytic to a host-associated state, L. monocytogenes relies on sensing host-specific signals that affect the transcription regulators CodY and PrfA and triggers the initiation of its virulence program [8,9,10,11,12]. In addition to these important regulatory proteins, it is clear that small regulatory RNAs (sRNAs), non-coding transcripts of about 50 to 500 nucleotides long, can contribute to the control of virulence gene expression at the post-transcriptional level [13,14].
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