Abstract
The parpAD1 locus was the first type I toxin–antitoxin (TA) system described in Gram-positive bacteria and was later determined to be the founding member of a widely distributed family of plasmid- and chromosomally encoded TA systems. Indeed, homology searches revealed that the toxin component, FstpAD1, is a member of the Fst/Ldr superfamily of peptide toxins found in both Gram-positive and Gram-negative bacteria. Regulation of the Fst and Ldr toxins is distinct in their respective Gram-positive and Gram-negative hosts, but the effects of ectopic over-expression are similar. While, the plasmid versions of these systems appear to play the canonical role of post-segregational killing stability mechanisms, the function of the chromosomal systems remains largely obscure. At least one member of the family has been suggested to play a role in pathogenesis in Staphylococcus aureus, while the regulation of several others appear to be tightly integrated with genes involved in sugar metabolism. After a brief discussion of the regulation and function of the foundational parpAD1 locus, this review will focus on the current information available on potential roles of the chromosomal homologs.
Highlights
The parpAD1 locus of the Enterococcus faecalis pheromone-responsive conjugative plasmid pAD1 was the first type I toxin-antitoxin (TA-1) system identified and characterized in Gram-positive bacteria [1,2,3]
The toxin and antitoxin RNAs are transcribed in opposite directions across a pair of DNA direct repeats, DRa and DRb, which provide a second region of complementarity between the RNAs
The purpose of this review is to provide a detailed comparison of the sequence, structure, regulation and effects of the toxins related to FstpAD1
Summary
The parpAD1 locus of the Enterococcus faecalis pheromone-responsive conjugative plasmid pAD1 was the first type I toxin-antitoxin (TA-1) system identified and characterized in Gram-positive bacteria [1,2,3]. Expanded examination of the surrounding sequences revealed that all of the structural features present in parpAD1 were highly conserved, including the convergent promoters, the bidirectional intrinsic terminator, the direct repeats and the regions of intramolecular complementarity in RNA I sequestering the 50 end and the SD sequence, suggesting that the mechanisms of regulation of toxin expression were broadly conserved. NMR structures of two Fst family members in membrane mimetics have been determined: the FstpAD1 prototype [18] and the PepA1 toxin of the S. aureus SprA1-SprA1AS locus [19] (Figure 2). PepA1 lacks the multiple acidic in opposite orientations must be considered These structural differences might account for the amino acids present in the Fst. C-terminus, so the possibility that the two toxins insert into the differences in the behavior of pAD1 the two toxins.
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