The regulatory region of natural isolates of simian virus 40 (SV40) is different from that of laboratory-adapted strains of the virus. The latter have a nucleotide sequence duplication within the enhancer region which varies slightly with each strain, whereas the duplication is lacking in fresh isolates of SV40, which contain an 'archetypal' regulatory region. Many isolates also display nucleotide differences in the DNA encoding the carboxy terminus of large tumour antigen (T-ag). To determine whether genetic changes in these two regions of the SV40 genome were detectable during laboratory adaptation and long-term passage, low-passage virus stocks of two laboratory strains which had detailed passage histories spanning more than 25 years (Baylor strain and VA45-54) were analysed using PCR, cloning and sequencing assays. Both laboratory and archetypal regulatory regions were present in low-passage stocks. Following duplication in the regulatory region, no additional changes were detectable. The variable region at the T-ag carboxy terminus did not undergo any change with tissue culture passage and may serve as a useful site for taxonomic classification of different strains of SV40. Cloned genomes containing single or duplicated enhancers derived from both SV40 strains were viable in CV-1 cells. Attempts to induce regulatory region duplications by 14 serial passages of SV40 archetypal strains in monkey cells were not successful. The results are compatible with tissue culture adaptation of SV40, reflecting either selection of a rare variant pre-existing in the original sample or generation of a rare regulatory region duplication in infected cells.


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