Deletion mutants of polyoma virus defining a nonessential region between the origin of replication and the initiation codon for early proteins.

Mutants of polyoma virus with deletions as large as 90 base pairs were isolated by selecting spontaneously arising genomes resistant to endonuclease HaeII or by treating HaeII- or BglI- cleaved linear DNAs with S1 nuclease and exonuclease III. All of the mutants were viable and, therefore, defined a nonessential region in the polyoma genome between the origin of DNA replication and the initiation codon for translation of early proteins. Several mutants with large deletions had altered growth properties, giving smaller plaques and lower virus yields than the parental wild-type virus. These viruses may lack sites that are important for DNA replication or for transcription and translation of early mRNA's. All of the mutants tested could transform BHK-21 cells to anchorage independence.