Walleye dermal sarcoma virus: OrfA N-terminal end inhibits the activity ofa reporter gene directed by eukaryotic promoters and has a negative effecton the growth of fish and mammalian cells

Walleye dermal sarcoma virus (WDSV) is a fish retrovirus causing a skin tum or termed walleye dermal sarcoma, which develops and regresses on a seasona l basis. The WDSV genome contains three short open reading frames designate d orfA, orfB, and orfC in addition to the viral structural genes, gag, pal, and env. orfA and orfB transcripts are detected in tumors by reverse trans cription-PCR. Recently, OrfA, whose amino acid sequence is similar to that of cyclins A and D, has been shown,vn to complement a cyclin-deficient yeas t strain. We report that expression of the accessory gene orfA inhibited no nspecifically the activity of a reporter gene directed by various eukaryoti c promoters. In addition, stable transfection with the wild-type orfA gener ated substantially fewer G418-resistant colonies in both fish and mammalian cells than the parent vector An orfA mutant expressing only the first N-te rminal 49 residues of the full-length protein had the same negative effect on the activity of the reporter gene and on the number of stably transfecte d colonies as the full-length OrfA. Thus, OrfA inhibits cell growth and/or causes cell death, and the first 49 N-terminal residues of this protein are sufficient to cause these negative effects.