Expression of polyoma virus middle-T antigen in Saccharomyces cerevisiae.

The polyoma middle-T gene, lacking its intron, was inserted into a yeast expression plasmid containing the phosphoglycerate kinase promoter. Such plasmids transformed yeast at low frequency and these transformants expressed middle-T antigen at a level of approximately 0.1% cell protein. Furthermore, expression of this protein was frequently lost during growth in liquid culture and this loss of middle-T was accompanied by a twofold increase in the rate of growth. The spontaneous production of a truncated middle-T antigen, lacking the C terminus, was also observed; the expression of this protein did not inhibit the growth rate of the cells. Recovery and analysis of the expression plasmids encoding the truncated molecule showed that a single C X G base pair had been deleted from a run of nine consecutive C X G base pairs (Pyr nucleotide 1239--1247) within the middle-T coding region. This frame-shift mutation results in premature termination of the protein and loss of the strongly hydrophobic region of the molecule believed to be responsible for the membrane association of middle-T antigen.