FIGURE 1 (a) The protein FBF-1 was identified in a three-hybrid screen. The hybrid RNA engineered to be the link between the binding protein MS2 and the (unknown) protein X–Gal4p fusion contained two PME sequences. (b) Activation of the his3-lacZ reporter gene led to production of β-galactosidase, which catalyzes the conversion of X-gal to a colored product. The color is seen only in cells expressing RNA containing PMEs of the proper sequence. The RNAs in other lanes of the gel provide controls that demonstrate specificity of binding. IRE is iron response element; A₃₀, a sequence of 30 A residues; and HIV-E, a 573-nucleotide RNA sequence derived from HIV.