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The large chlorella virus PBCV-1, which contains double-stranded DNA (dsDNA), en¬codes a 94-codon open reading frame (ORF) that contains a motif resembling the signature se¬quence of the pore domain of potassium channel proteins. Phylogenetic analyses of the encoded protein, Kcv, indicate a previously unidentified type of potassium channel. The messenger RNA encoded by the ORF leads to functional expression of a potassium-selective conductance in Xen¬opus laevis oocytes. The channel blockers amantadine and barium, but not cesium, inhibit this conductance, in addition to virus plaque formation. Thus, PBCV-1 encodes the first known viral protein that functions as a potassium-selective channel and is essential in the virus life cycle.