Genes making up superantigen pathogenicity islands (SaPIs) can commandeer invading phage proteins and use them to disconnect from the genomes of staphylococcal cells, package themselves into highly transmissible particles, and then escape the infected bacteria, according to José R. Penadés from the Centro de Investigatiòn y Tecnologia Animal in Segorbe, Castellón, Spain. This activity is part of a previously unrecognized mechanism for transferring genes horizontally among microbes-one with potentially adverse public health consequences.

"The SaPI is trying to abandon ship," says Ray Schuch from Rockefeller University in New York, N.Y., who was not involved in the research. "Once the bacterium has been infected by a phage, a so-called ‘helper' phage in this case, it's not long for the world; it will not be replicating again, ever." The mechanism that Penadés and his collaborators describe, and which Schuch calls "elegant," helps to explain how the genes within a SaPI sense phage and then mobilize, he adds.

The coopted phage proteins are called "moonlighters" because their primary functions differ considerably from the role that they play when SaPIs exploit them. This shift is "a remarkable evolutionary adaptation," Penade´ s notes. Details of their research appear in the 10 June 2010 Nature (doi:10.1038/nature 09065).

SaPIs are discrete, chromosomally integrated units containing about 15 kb of DNA that reside on specific bacterial chromosomal sites and which, ordinarily, excise and replicate only in the presence of phages. "After replication, they're encapsidated at high efficiency into highly mobile phage-like infectious particles," says Richard P. Novick, a collaborator in this study, who is at New York University School of Medicine in New York, N.Y. Stl, a repressor protein encoded within the "island," ordinarily holds the pathogenicity islands in place, acting much like classic prophage repressors but, in this case, tethering the superantigen-encoding genes to the bacterial genome. "The general mechanism of phage-induced SaPI derepression involves proteins that function as antirepressors, complexing with Stl to prevent it from binding DNA," says María Ángeles Tormo-Más, lead scientist on this study. Earlier, Novick and Penadés proved the importance of Stl by showing that, when stl is inactivated, SaPI excises and replicates in the absence of inducing phages.

When helper phage 80α particles infect SaPI-containing Staphylococcus aureus cells, the phages mobilize at least five different SaPIs, each using a different Stl protein. That activity "raised the question of how one phage can have such broad specificity," Tormo-Más says. When the scientists tested three S. aureus pathogenicity islands, SaPI1, SaPIbov1, and SaPIbov2, they learned that each island uses a different nonessential 80α protein for its own derepression.

Reasoning that phages with proteins immune to SaPI highjacking would more readily lyse island-carrying S. aureus and form easily discernible plaques, the researchers began studying plaque-forming 80α mutants in SaPIbov1-containing bacterial strains. "The mutant phages had lost the ability to mobilize [pathogenicity] islands, and this [loss] was consistent with failure to relieve Stl-mediated repression," Penadés says.

Among 18 independent SaP1bov1- resistant 80α mutants, the changes enabling plaque formation are all located in a single phage gene, dut, which encodes dUTPase. "In addition to eliminating dUTP," Penade´s says, "some dUTPases induce SaPIbov1 replication, indicating that Dut is a true bifunctional ‘moonlighting' protein because the two functions are unrelated and some mutations affect one activity but not the other."

Notably, SaPIbov1 can be transferred to staphylococci other than S. aureus, including S. xylosus, S. chromogenes, S. epidermidis, and S. intermedius. Both SaPI1 and SaPIbov1 are also found in Listeria monocytogenes. "The staphylococcal phages that induce SaPI transfer to L. monocytogenes don't form plaques-thus, silently spreading toxicity islands may represent a new and important mechanism of horizontal gene transfer among bacteria," says Novick. He also notes that by impairing phage reproduction, SaPIs are a fitness asset to the host bacteria.

Furthermore, the Penadeés team contends that phage-related chromosomal islands such as SaPIs have evolved from prophages in a highly specific manner and represent an extension of the continuum leading from fully lytic to temperate phages. It is worrisome, the researchers note, that SaPIs are highly mobile and the only known repository of several superantigen genes, including tst, seb, and sec, and could therefore play a major role in disseminating genes that make humans sick.

Marcia Stone
Marcia Stone is a science writer based in New York City. More of her work can be seen on http://www.mstoneworks.net.