title: Characterization of a Streptococcus mutans intergenic region containing a small toxic peptide and its cis-encoded antisense small RNA antitoxin.
abstract: Toxin-antitoxin (TA) modules consist of a pair of genes that encode two components: a protein toxin and an antitoxin, which may be in the form of either a labile protein or an antisense small RNA. Here we describe, to the best of our knowledge, the first functional chromosomal type I TA system in streptococci. Our model organism is the oral pathogen Streptococcus mutans. Our results showed that the genome of S. mutans UA159 reference strain harbors a previously unannotated Fst-like toxin (Fst-Sm) and its cis-encoded small RNA antitoxin (srSm) converging towards the end of the toxin gene in IGR176, a small intergenic region of 318 nt. Fst-Sm is a small hydrophobic peptide of 32 amino acid residues with homology to the Fst toxin family. Transcripts of ∼200 nt and ∼70 nt specific to fst-Sm mRNA and srSm RNA, respectively, were detected by Northern blot analysis throughout S. mutans growth. The toxin mRNA was considerably more stable than its cognate antitoxin. The half-life of srSm RNA was determined to be ∼30 min, while fst-Sm mRNA had a half-life of ∼90 min. Both fst-Sm and srSm RNAs were transcribed across direct tandem repeats providing a region of complementarity for inhibition of toxin translation. Overproduction of Fst-Sm had a toxic effect on E. coli and S. mutans cells which can be neutralized by coexpression of srSm RNA. Deletion of fst-Sm/srSm locus or overexpression of Fst-Sm/srSm had no effect on S. mutans cell growth in liquid medium and no differences in the total biofilm biomass were noted. In contrast, mild-overproduction of Fst-Sm/srSm type I TA system decreases the levels of persister cells tolerant to bacterial cell wall synthesis inhibitors.
authors: Dental Research Institute, Faculty of Dentistry, University of Toronto, Toronto, Ontario, Canada.
authors: Stephanie
authors: S
authors: Koyanagi
authors: Céline M
authors: CM
authors: Lévesque
mesh_terms: D000595:Amino Acid Sequence; D000992:Antitoxins / Q000235:genetics*; D001426:Bacterial Proteins / Q000235:genetics*; D001427:Bacterial Toxins / Q000235:genetics*; D021901:DNA, Intergenic; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D016372:RNA, Antisense / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics; D013295:Streptococcus mutans / Q000235:genetics* / Q000472:pathogenicity
chemical_list: D000992:Antitoxins; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D021901:DNA, Intergenic; C582269:Fst-like toxin, Streptococcus mutans; D016372:RNA, Antisense; D012333:RNA, Messenger