Decreasing the number of 68-base-pair tandem repeats in the BK virus transcriptional control region reduces plaque size and enhances transforming capacity.

Wild-type BK virus, which has three 68-base-pair (bp) elements tandemly repeated in its transcriptional control region, forms clear, large plaques in human embryonic kidney cell cultures but rarely transforms hamster or rat cells. From this BK virus DNA, deletion mutants with fewer than three 68-bp elements were constructed and characterized. The mutant DNA with two 68-bp elements formed small, turbid plaques in human cell cultures and transformed rat 3Y1 cells inefficiently. The mutant DNA with one 68-bp element formed minute, turbid plaques in human cell cultures and transformed rat cells efficiently.