Toxic Relationships: Discovery and Characterization of a Virally Encoded Toxin in the Thermophilic Archaeal Fusellovirus SSV1

Mechanisms for maintenance of chronic viruses are poorly understood, particularly for archaeal viruses. Here, we identify the product of Sulfolobus spindle-shaped virus 1 (SSV1) ORF a291 as a virally-encoded toxin required for growth inhibition but dispensable for viral replication and virion production. Viruses lacking ORF a291 replicated their genomes and formed morphologically normal spindle-shaped particles yet failed to inhibit growth of uninfected Saccharolobus solfataricus. Substitution of residues at a predicted N-terminal signal peptide cleavage site abolished growth suppression without affecting replication, suggesting that secretion is essential for toxin function. Despite primary sequence divergence among fusellovirus toxin candidates, analysis of protein structure predictions revealed a conserved hydrolase-like fold across SSV1, SSV9 and SSV10 toxins. These findings demonstrate functional separation of viral replication and host growth suppression and support a model in which chronic archaeal viruses modulate host competition through antagonistic factors. This work expands the known diversity of viral toxins and suggests that fuselloviruses employ conserved strategies to promote persistence in extreme environments. Impact Statement: This work identifies a virally-encoded toxin, the product of ORF a291, in Sulfolobus spindle-shaped virus 1. Unlike most toxins, this toxin appears to be secreted and only affects uninfected cells. Our findings expand the known diversity of viral toxins and suggest that Fuselloviruses broadly employ these mechanisms to promote their persistence in extreme environments.