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Here, we identify the product of Sulfolobus spindle-shaped virus 1 (SSV1) ORF a291 as a putative 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 the 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 putative viral toxins and suggests that fuselloviruses employ conserved strategies to promote persistence in extreme environments.<\/jats:p>","DOI":"10.3390\/v18070802","type":"journal-article","created":{"date-parts":[[2026,7,21]],"date-time":"2026-07-21T16:16:39Z","timestamp":1784650599000},"page":"802","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Toxic Relationships: Characterization of a Putative Virally Encoded Toxin in the Thermophilic Archaeal Fusellovirus SSV1"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1814-6203","authenticated-orcid":false,"given":"Jonathan C.","family":"Abshier","sequence":"first","affiliation":[{"name":"Center for Life in Extreme Environments and Biology Department, Portland State University, Portland, OR 97201, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Patrizia L.","family":"Alpapara","sequence":"additional","affiliation":[{"name":"Center for Life in Extreme Environments and Biology Department, Portland State University, Portland, OR 97201, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Guas\u00e5li","family":"Tomokane","sequence":"additional","affiliation":[{"name":"Center for Life in Extreme Environments and Biology Department, Portland State University, Portland, OR 97201, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6086-0238","authenticated-orcid":false,"given":"Kenneth M.","family":"Stedman","sequence":"additional","affiliation":[{"name":"Center for Life in Extreme Environments and Biology Department, Portland State University, Portland, OR 97201, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1511","DOI":"10.1038\/ismej.2017.16","article-title":"Lysogeny in nature: Mechanisms, impact and ecology of temperate phages","volume":"11","author":"Hargreaves","year":"2017","journal-title":"ISME J."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"689","DOI":"10.1038\/nature19366","article-title":"Ecogenomics and potential biogeochemical impacts of globally abundant ocean viruses","volume":"537","author":"Roux","year":"2016","journal-title":"Nature"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"801","DOI":"10.1038\/nrmicro1750","article-title":"Marine viruses\u2014Major players in the global ecosystem","volume":"5","author":"Suttle","year":"2007","journal-title":"Nat. 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