We compared the nucleotide sequence in the transcriptional control region of BK virus isolates cloned directly from human urine (BK-WW) with that of prototype BK virus. BK-WW was found to have a 63-base-pair insertion and only one of the 68-base-pair enhancer repeat elements. In transient expression assays, BK-WW enhancer showed approximately one-half the activity given by the prototype enhancer.
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