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[Position of the attachment site of the SV40 virus genome at the transcription unit level].

DNA of OV40-transformed 3T3 cell line is more methylated than that of normal cells. Integrated sequences of OV40 DNA, however, are methylated less than the host DNA. Virus-specific sequences are flanked by highly methylated areas of cellular DNA of intron nature. The virus-specific pre-mRNA isolated from nuclei (28-37 S) is capable of hybridization with the low-methylated region of early OV40 genes coding for T-antigen, highly methylated exon containing a signal for polyadenylation and stop-codon. After processing, the virus-specific mRNA (19 S) extracted from polysomes loses the capacity of hybridization with highly methylated intron apparently due to splicing. On the other hand, the early gene of the virus is mapped in the A fragment obtained by combined hydrolysis of OV40 DNA with ECO RI, BamHI and Bg III. Because this fragment is shorter (2.6 Kb) than virus-specific mRNAs, it is assumed that T-antigen (mol. mass 94,000) is a hybrid protein containing amino acid sequences coded for by cellular DNA. From this point of view, integration of OV40 DNA leads to the formation of a "virus-cell transcription unit" for T-antigen, and multiple integration may be associated with repetition of the same target.