Abstract
The work of Kwong, Jensen and Firth (above) dramatically expands the pAD1 par-like family of RNA-regulated toxin–antitoxin (TA) systems described in our recent study (Weaver et al., 2009). While the primary sequences of these systems are highly diverse, important interacting and regulatory structures are clearly conserved. These results would appear to contradict an earlier report indicating that, while several proteic TA systems are widespread among enterococci, par-like sequences are rare (Moritz & Hergenrother, 2007). However, given the sequence diversity of the par-like systems identified, it is not surprising that the PCR-based screening approach used by Moritz & Hergenrother (2007) failed to amplify any of the par-like loci. This information should be considered in future surveys designed to determine the distribution of other type I TA systems.
Our work and that described above should also be viewed in the larger context of RNA-regulated, probably membrane-localized, small peptide toxins, which were originally reviewed by Fozo et al. (2008). While the number of Gram-positive representatives in their sample was limited, it is now clear that such peptides are widespread in Gram-positive bacteria and are likely to play roles at least as important as their counterparts in Gram-negative bacteria. Finally, while most of the par-like loci appear to be associated with mobile elements, the association of some of the chromosomally located loci with genes involved in basic carbon metabolism is intriguing. Since the function of many chromosomally encoded TA systems has been elusive and controversial, it would be worth determining if regulation of these elements is integrated with basic cellular metabolism.