CODING SEQUENCES OF BOTH GENOME SEGMENTS OF A EUROPEAN VERY VIRULUNT INFECTIOUS BURSAL DISEASE VIRUS

The sequences of segment A (encoding the VP2-VP4-VP3 polyprotein and V P5) and segment B (encoding VP1) of a recent, 'very virulent' (VV) Eur opean isolate (UK661) of infections bursal disease virus (IBDV), a bir navirus, have been determined There are 26 to 36 amino acid substituti ons compared to any other type I IBDV within the segment A polyprotein (of these, 15 are unique) and about 50 substitutions within VP1 (of w hich 16 are unique) There is more variation compared to classical and antigenic variant viruses, of both virulent and attenuated phenotype, in VP1, VP3 and VP4 than in VP2, even though the latter has previously been identified as tile most variable protein between different strai ns of type I IBDV In VP3 and VP4, UK661 is the most diverged type I IB DV. Thus the origin of the virus is unclear. It is possible that stron g functional constraints have preferentially maintained the primary st ructure of VP2, though the Possibility of recombination cannot be excl uded There are no clear candidate mutations to account for the enhance d virulence of the VV IBDV polymerase; motifs are well conserved in VP 1 but there is an amino acid substitution next to the predicted active -site serine of the viral protease (VP4). In addition, there is a cons ervative substitution close to the postulated VP2-VP4 cleavage site. I t is also now apparent that sequences of IBDV segment B (the segment e ncoding the RNA polymerase) do not group according to serotype (specif ied by the capsid proteins encoded on segment A), indicating that segm ent reassortment has occured.