Transcription of a defective polyoma virus genome

The circular genome of the cloned defective polyoma virus D-50 consists of tandemly repeated copies of the DNA sequence between 67 and 84 units on the wild-type polyoma virus DNA map. Each repeated copy thus contains the origin of viral DNA replication, which is located at about 71 map units. Viral RNA was synthesized in vitro using viral transcription complexes extracted late (30 hr) after infection from mouse cells co-infected with D-50 and helper wild-type virus. Both wild-type and D-50 DNA molecules were active as templates for in vitro transcription. Approximately 84% of the RNA transcribed in vitro from wild-type DNA was complementary to the L DNA strand. This is normal for wild-type transcription late after infection. By contrast, at least 90% of the RNA transcribed from D-50 DNA molecules was complementary to the E DNA strand. After normalization of the data to account for the observed molar ratio of D-50 DNA repeated sequences to unit length wild-type DNA, we estimate that transcription of the E DNA strand of each D-50 repeated unit is about 1.4 times as efficient as transcription of the wild-type E DNA strand. Transcription of the D-50 L DNA strand, however, is only 0.03 times as efficient as transcription of the wild-type L DNA strand. The implications of these results concerning the nature and location of promoter sequences in polyoma DNA are discussed.