Characterization of viable mutants of polyomavirus cold sensitive for maintenance of cell transformation.

We mutagenized a cloned fragment of polyoma DNA encoding portions of the middle size (MT) and large T antigens. We regenerated infectious viral genomes containing the mutagenized DNA and tested their transforming ability at 32 and 39 degrees C. We isolated three nontransforming mutants and two mutants which were cold sensitive for the maintenance of cell transformation. The nontransforming mutants contained amber termination codons in the reading frame for the MT antigen. They synthesized truncated MT antigens which lacked MT-associated protein kinase activity. The cold-sensitive mutants synthesized MT antigens indistinguishable from wild type with regard to size, stability at 32 and 39 degrees C, intracellular location, and associated protein kinase activity. One of the mutants was shown by nucleotide sequence analysis to contain a single amino acid change in the MT antigen, located two residues upstream from the C-terminal hydrophobic region, and no changes in the large T antigen. The other mutant contained two amino acid changes in the MT antigen and two amino acid changes in the large T antigen.