A novel, highly regulated, rapidly inducible system for the expression of chicken progesterone receptor, cPRA, in Saccharomyces cerevisiae

A rapidly inducible and tightly regulated system for the expression of protein in yeast is based on a chimeric promoter constructed of two copies of a vitellogenin-estrogen-response element (ERE) which are inserted upstream from the promoter of the yeast gene encoding iso-1-cytochrome c. The chimeric promoter was inserted in a yeast expression plasmid upstream from the coding sequence of ubiquitin fused in frame to a cDNA encoding the full-length chicken progesterone receptor A (cPRA). The resultant plasmid (YEpA2) was co-transformed in Saccharomyces cerevisiae with a plasmid which encodes the human estrogen receptor. Estradiol (E2)-induced transactivation of the chimeric promoter results in transcription of the cPRA gene from YEpA2, and synthesis of cPRA. The fusion protein, ubiquitin-cPRA, is rapidly cleaved in vivo to produce cPRA. Analysis of samples by Western immunoblot shows that cPRA is almost undetectable in the absence of E2, and that treatment with 50 nM E2 results in a 500-1000-fold induction of cPRA (0.06-0.3% of the total protein) after 1 h. The plasmid-expressed soluble receptor is stable and demonstrates the correct affinity for its ligand. We have prepared yeast extracts using enzymatic digestion of the cell wall with oxalyticase followed by hypotonic shock. This has resulted in a dramatic increase in the % of receptor which binds hormone compared to previous studies which used mechanical disruption techniques. The cPRA is biologically active since it activates transcription of a co-transformed reporter gene containing its response element.(ABSTRACT TRUNCATED AT 250 WORDS)