Four operational yeast cellular À1 RF signals derived from the BUB3, EST2, SPR6 and TBF1 genes were employed to test the hypothesis that À1 RF signals function as mRNA destabilization elements. The slippery heptamers for these À1 RF signals begin at nucleotides 858, 1653, 279 and 1521 of their respective ORFs. These were cloned into a yeast PGK1 reporter gene so that frameshifted ribosomes are directed to PTCs. All inserts were flanked by sequences derived from Renilla and firefly luciferase genes, providing unique exogenous sequences for specific detection of the reporter mRNAs. Two additional PGK1 reporters without À1 RF signals were used as controls: a readthrough reporter encoded a continuous ORF, while a PTC control contained an in-frame UAA termination codon ( Figure 1 ). Reporters were introduced into wild-type yeast cells; their steady state mRNA abundances were determined by RNA blot analysis and normalized to U3 snoRNA controls ( Figure 2) . A Figure 1 . Schematic of PGK1 reporter vectors used to monitor the effects of À1 RF signals on mRNA stability. The indicated Renilla and firefly luciferase derived sequences from pJD375 were cloned into the unique KpnI restriction site in a high copy PGK1 expression vector to create the readthrough control (pJD753). The indicated À1 RF signals derived from BUB3, EST2, SPR6 and TBF1 were cloned into SalI/BamHI digested pJD753. Colored arcs depict computationally predicted base-paired stems (17) . The premature termination control (PTC) was constructed by mutagenizing pJD753 to create an in-frame TAA codon. minimum of three independent blots were performed for all experiments. We note that the blots shown in Figure 2 are simplified for the purpose of publication and that the strains are not all isogenic with one another. In contrast, the bar graphs shown in Figure 2 represent data summarized from multiple blots using isogenic strains.