Significance Symbioses between animals and microbes are now recognized as critical to many aspects of host health. This is especially true in insects, which are associated with diverse maternally transmitted endosymbionts that can protect against parasites and pathogens. Here, we find that Spiroplasma—a defensive endosymbiont that protects Drosophila during parasitism by a virulent and common nematode—encodes a protein toxin, a ribosome-inactivating protein (RIP) related to bacterial virulence factors such as the Shiga-like toxins in Escherichia coli. We further find that nematode ribosomal RNA suffers depurination consistent with attack by a RIP when the host is protected by Spiroplasma, suggesting a mechanism through which symbiotic microbes may protect their hosts from disease. Vertically transmitted symbionts that protect their hosts against parasites and pathogens are well known from insects, yet the underlying mechanisms of symbiont-mediated defense are largely unclear. A striking example of an ecologically important defensive symbiosis involves the woodland fly Drosophila neotestacea, which is protected by the bacterial endosymbiont Spiroplasma when parasitized by the nematode Howardula aoronymphium. The benefit of this defense strategy has led to the rapid spread of Spiroplasma throughout the range of D. neotestacea, although the molecular basis for this protection has been unresolved. Here, we show that Spiroplasma encodes a ribosome-inactivating protein (RIP) related to Shiga-like toxins from enterohemorrhagic Escherichia coli and that Howardula ribosomal RNA (rRNA) is depurinated during Spiroplasma-mediated protection of D. neotestacea. First, we show that recombinant Spiroplasma RIP catalyzes depurination of 28S rRNAs in a cell-free assay, as well as Howardula rRNA in vitro at the canonical RIP target site within the α-sarcin/ricin loop (SRL) of 28S rRNA. We then show that Howardula parasites in Spiroplasma-infected flies show a strong signal of rRNA depurination consistent with RIP-dependent modification and large decreases in the proportion of 28S rRNA intact at the α-sarcin/ricin loop. Notably, host 28S rRNA is largely unaffected, suggesting targeted specificity. Collectively, our study identifies a novel RIP in an insect defensive symbiont and suggests an underlying RIP-dependent mechanism in Spiroplasma-mediated defense.
P. Hamilton, Fangni Peng, M. Boulanger
Proceedings of the National Academy of Sciences