HOST RESTRICTION IN THE PRODUCTIVE CYCLE OF AVIAN POLYOMAVIRUS BUDGERIGAR FLEDGLING DISEASE VIRUS TYPE-3 DEPENDS ON A SINGLE AMINO-ACID CHANGE IN THE COMMON REGION OF STRUCTURAL PROTEINS VP2 VP3/

The three avian polyomaviruses budgerigar fledgling disease virus type s 1 to 3 (BFDV-1 to -3) contain genomes of identical size, 4981 bp. Wi th differences of up to only 15 bp between the three genomes, these vi ruses show distinct tropism for cultured cells of various avian specie s: infection of chicken embryo (CE) cells with BFDV-1 and -2 results i n virus propagation, whereas BFDV-3 is not replicated; all three virus es replicate, with different efficiencies, in infected Muscovy duck ce lls. Transfection of CE cells with BFDV-3 DNA results in a single prod uctive cycle. As shown by construction of hybrid virus genomes and sit e-directed mutagenesis, a single amino acid difference (glycine instea d of valine or alanine) within the common region of the minor structur al proteins VP2/VP3 is responsible for this type of abortive infection of CE cells. Further experiments indicate a defect in one of the earl y steps during infection, at or prior to uncoating.