High-level replication of polyomavirus BK (BKV) in kidney transplant recipients is associated with the emergence of BKV variants with rearranged (rr) non-coding control region (NCCR) increasing viral early gene expression and cytopathology. Cloning and sequencing revealed the presence of a BKV quasispecies which included non-functional variants when assayed in a recombinant virus assay. Here we report that the rr-NCCR of BKV variants RH-3 and RH-12, both bearing a NCCR deletion including the 5' end of the agnoprotein coding sequence, mediated early and late viral reporter gene expression in kidney cells. However, in a recombinant virus they failed to produce infectious progeny despite large T-antigen and VP1 expression and the formation of nuclear virus-like particles. Infectious progeny was generated when the agnogene was reconstructed in cis or agnoprotein provided in trans from a co-existing BKV rr-NCCR variant. We conclude that complementation can rescue non-functional BKV variants in vitro and possibly in vivo.
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