One of the features distinguishing SARS-CoV-2 from its more pathogenic counterpart SARS-CoV is the presence of premature stop codons in its
ORF3b
gene. Here, we show that SARS-CoV-2 ORF3b is a potent interferon antagonist, suppressing the induction of type I interferon more efficiently than its SARS-CoV ortholog. Phylogenetic analyses and functional assays reveal that SARS-CoV-2-related viruses from bats and pangolins also encode truncated
ORF3b
gene products with strong anti-interferon activity. Furthermore, analyses of approximately 17,000 SARS-CoV-2 sequences identify a natural variant in which a longer
ORF3b
reading frame was reconstituted. This variant was isolated from two patients with severe disease and further increased the ability of ORF3b to suppress interferon induction. Thus, our findings not only help to explain the poor interferon response in COVID-19 patients but also describe the emergence of natural SARS-CoV-2 quasispecies with an extended
ORF3b
gene that may potentially affect COVID-19 pathogenesis.
Highlights d SARS-CoV-2 ORF6 inhibits innate immune signaling via its C-terminal region d ORF6 proteins of SARS-CoV-2 lineages are more potent than those of SARS-CoV d Residues E46 and Q56 determine the anti-interferon activity of ORF6 d The ORF6 gene is truncated C-terminally in 0.2% of SARS-CoV-2 isolates
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