Abstract
Plasmids carry genes that give bacteria beneficial traits and allow them to survive in competitive environments. In many cases, they also harbor toxin-antitoxin (TA) systems necessary for plasmid maintenance. TA systems are generally characterized by a stable “toxin”, a protein or peptide capable of killing the cell upon plasmid loss and by an unstable “antitoxin”, a protein or a non-coding RNA that inhibits toxin activity. Here we report data toward the identification of a RNA-regulated TA system in the plasmid DNA of L. rhamnosus isolated from cheese. The proposed TA system comprises two convergently transcribed RNAs: a toxin RNA encoding a 29 amino acid peptide named Lpt and an antitoxin non-coding RNA. Both toxin and antitoxin RNAs resulted upregulated under conditions mimicking cheese ripening. The toxicity of the Lpt peptide was demonstrated in E. coli by cloning the Lpt ORF under the control of an inducible promoter. Bioinformatics screening of the bacterial nucleotide database, shows that regions homologous to the Lpt TA locus are widely distributed in the Lactobacillus genus, particularly within the L. casei group, suggesting a relevant role of TA systems in plasmid maintenance of cheese microbiota.
Highlights
Lactobacillus rhamnosus is a non-starter lactic acid bacterium that plays a significant role during cheese ripening, contributing to the formation of flavor
PR1473 grown in MRS and Cheese Broth (CB) media evaluated by using cDNA-AFLP with different primer sets
In this work we describe how the detection of an overexpressed RNA by transcriptomic analysis, lead us to the identification of a putative type I TA system in the plasmid DNA of two L. rhamnosus strains
Summary
Lactobacillus rhamnosus is a non-starter lactic acid bacterium that plays a significant role during cheese ripening, contributing to the formation of flavor. When a plasmid-free variant is produced, owing to a replication error or to defects in plasmid maintenance, a rapid depletion of the antitoxin occurs in newborn plasmid-free cells Under these conditions, the stable toxin inherited from the mother cell no longer neutralized by the antitoxin, causes cell death[13]. The antitoxin RNA harbours a 24 nt sequence complementary to a region encompassing the start codon of the toxin mRNA Both toxin and antitoxin RNA are upregulated in a cheese-like medium, suggesting a possible role under stress conditions. Bioinformatics analysis shows that regions homologous to the proposed TA locus are widely distributed in plasmids harbored by the L. casei group (L. rhamnosus, L. casei and L. paracasei) and by L. brevis, suggesting a relevant function of TA systems in the Lactobacillus genus
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