Pathogenicity islands have a major role in spreading virulence genes among bacterial populations. A notable example are the phage-related pathogenicity islands of staphylococci, the SaPIs, which are responsible for the inter- as well as intrageneric spread of toxins – such as TSST-1 (toxic shock syndrome toxin) and other superantigens – through the exploitation of specific staphylococcal helper phages for high-frequency transfer within phage-encoded particles. Toxic shock syndrome is a rare, potentially fatal illness that can be caused by the release of toxins from Staphylococcus. The toxic particles are encoded by discrete genetic units called pathogenicity islands, which reside passively in the host chromosome, under the control of the global repressor Stl, unless activated by a helper phage. This paper shows that a non-essential and specific protein from the helper phage 80α is responsible for de-repression of the pathogenicity island, providing the mechanism for the first step of its mobilization. The proteins involved are ‘moonlighters’, because they have two different and genetically distinct activities. Through a remarkable evolutionary adaptation, various related pathogenicity islands co-opt entirely unrelated phage proteins to aid in their mobilization.
Moonlighting bacteriophage proteins derepress staphylococcal pathogenicity islands. 2010 Nature. 465(7299): 779-782
Staphylococcal superantigen-carrying pathogenicity islands (SaPIs) are discrete, chromosomally integrated units of ~15 kilobases that are induced by helper phages to excise and replicate. SaPI DNA is then efficiently encapsidated in phage-like infectious particles, leading to extremely high frequencies of intra- as well as intergeneric transfer. In the absence of helper phage lytic growth, the island is maintained in a quiescent prophage-like state by a global repressor, Stl, which controls expression of most of the SaPI genes4. Here we show that SaPI derepression is effected by a specific, non-essential phage protein that binds to Stl, disrupting the Stl–DNA complex and thereby initiating the excision-replication-packaging cycle of the island. Because SaPIs require phage proteins to be packaged, this strategy assures that SaPIs will be transferred once induced. Several different SaPIs are induced by helper phage 80α and, in each case, the SaPI commandeers a different non-essential phage protein for its derepression. The highly specific interactions between different SaPI repressors and helper-phage-encoded antirepressors represent a remarkable evolutionary adaptation involved in pathogenicity island mobilization.