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Evolutionary changes of transcriptional control region in a minute-plaque viable deletion mutant of BK virus

Two plaque morphology BK virus (BKV) mutants (pm526 and pm527) rescued from a hamster pineocytoma cell line Pc13 were characterized and compared with a similarly rescued and previously characterized mutant (pm522), its derivatives (tr530, tr531, and tr532), and the wild type (501) for their biological activities and for the structures of their transcriptional control regions. The two mutants grew somewhat more slowly in human embryonic kidney cells but transformed rat 3Y1 cells more efficiently than did the wild-type BKV. BKV pm526 formed minute plaques and had the shortest transcriptional control region, having only one 68-base-pair element, which is triplicated in the wild-type BKV. BKV pm526 was unstable during repeated replication in human embryonic kidney cells and yielded large-plaque viruses with longer HindIII C segments encompassing the BKV DNA replication origin. Comparison of nucleotide sequences of the transcriptional control regions among the mutants and the wild type showed that pm526 is a parent virus, from which all the other mutants had evolved, and that the evolutionary changes of the plaque size, from minute to small to large, were due to duplications of a certain segment containing the adenovirus E1A enhancer core or the simian virus 40 enhancer core found in the BKV 68-base-pair element. The activities to enhance early transcription, as measured by the ability to direct the synthesis of chloramphenicol acetyltransferase, approximately paralleled the plaque size. The duplication containing the adenovirus E1A enhancer core did not affect the transforming capacity of the parent virus, but the duplication including the simian virus 40 enhancer core significantly lowered the transforming capacity for rat cells.