Deletions of N-terminal sequences of polyoma virus T-antigens reduce but do not abolish transformation of rat fibroblasts.

Polyoma virus transforms, upon infection or DNA transfection, nonpermissive Fisher rat fibroblasts. Cloned viral DNA was deleted of sequences around the Bg/I site at nucleotide 86 by Bal31 nuclease treatment and then recloned in Escherichia coli. The extent of deletion for each mutant was then determined by DNA sequencing. Deletions included the early transcription control signals; others stretched into the N-terminal coding sequences of the viral tumor antigens. The transformation efficiency of 16 mutants was tested by transfecting rat fibroblasts. Expression of the T antigens was analyzed by immunofluorescence detection after transfection of rat fibroblasts, mouse secondary embryo cells, and HeLa cells. We found that the absence of the early transcription control sequences (TATA and CAAT boxes) did not significantly alter the transformation capacity of the virus. On the other hand, deletion of the initiator methionine ATG codon or further into the coding sequences did abolish the transformation capacity in some mutants, whereas others maintained a reduced transforming activity, possibly by initiation of translation in a penultimate methionine.