Fig. 5. Conditional expression of Ptip1 under the GAL promoter. (A) Schematic representation of the hybrid construct introduced at the PTI1 chromosomal locus (Lafontaine and Tollervey, 1996). The Pti1 protein is tagged at the N-terminus with the c-myc epitope. Its expression is induced in galactose-containing medium and repressed in glucose. The black box represents the c-myc epitope tag. (B) Cleavage and polyadenylation assays were performed with extracts prepared with the GAL::(c-myc)-Pti1p-expressing strain as in Figure 4B. (C) Immunoblot analysis of the GAL and GLU extracts with monoclonal antibodies to the c-myc epitope tag (1:2000), and with polyclonal antibodies to Rna15p and Pap1p as in Figure 4C.