Regulatory mutants of polyoma virus defective in DNA replication and the synthesis of early proteins

In polyoma virus the origin of replication, the 5′ ends of early mRNAs, and the initiation codon for early protein synthesis map within an approximately 200 bp region of the genome. We have previously reported the isolation and partial characterization of viable mutants of polyoma virus with deletions in this important regulatory region of the genome. Three of the mutants with large deletions, one of which had significantly altered growth properties, have been further characterized with respect to their nucleotide sequence alterations and their levels of viral DNA replication and of early protein synthesis. The nearly coincident deletions in mutants 17 and 2–19 reduce the capacity of these viruses to replicate, even in the presence of a coinfecting virus; thus they help define one boundary of the origin of DNA replication. The deletion in mutant 75 appears to remove sequences that are essential for efficient expression of early genes, but has little or no effect upon DNA replication. Its defect is complemented in trans by wild-type virus. All three mutants eliminate sequences which are candidates for RNA polymerase and ribosome binding sites near the initiation codon for early proteins.