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Two major subclasses of simian virus 40 (SV40) large T antigen were separated by zone velocity sedimentation of crude extracts from productively infected cells. These subclasses, which have been shown to differ biologically and biochemically (Fanning et al., 1981), sedimented at 5‐6S and 14‐16S. The amount of T antigen in each form was estimated by complement fixation and by immunoprecipitation of T antigen from extracts of cells chronically labeled with [35S]methionine. Each form of T antigen was tested for specific binding to end‐labeled restriction fragments of SV40 DNA using an immunoprecipitation assay. The 5‐6S and 14‐16S forms of T antigen both bound specifically to DNA sequences in the SV40 HindIII C fragment. The sequences required for binding both forms were localized in the same 35‐bp region of the origin. However, significant differences in binding activity and affinity for specific and nonspecific DNA were demonstrated. These properties suggest that T antigen subclasses may serve different functions in the lytically infected cell.

We have used differential cDNA cloning techniques to isolate a number of genes that are activated as a result of transformation by the DNA tumour virus Simian virus 40. From the nucleotide sequences of the cDNA clones we have been able to identify some of these genes. One of them derives from the major histocompatibility complex and contains a repetitive element that identifies a large number of RNAs present in pluripotential embryonic cells.

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