FIG. 7.
Synthesis of both FLI-1 isoforms is positively regulated by the conserved UAA +15 and UGA +35 stop codons. The same experiment as in Fig. 6 was performed using the different mRNA shown in panel A, in which the overlap of the two 5′ uORFs initiated at either AUG −41 (hatched boxes) or GUG −37 (grey boxes) and the FLI-1 ORF (open boxes) was progressively increased by the mutations of the corresponding in-frame stop codons (crosses). These stop codons are numbered from 1 to 8 depending on their increasing distance from AUG +1. Stop codons S1 (UAA +15, changed to CAA), S3 (UGA +39, changed to CGA), S4 (UGA +57, changed to CGA) S6 (UGA +120, changed to CGA) and S8 (UGA +210) are located in the same frame as the GUG −37 codon, whereas stop codons S2 (UGA +35, changed to UCA), S5 (UGA +101, changed to UGC), and S7 (UGA +185) are located in the same frame as AUG −41 codon. (B) Autoradiogram of the gel showing the different proteins synthesized; the positions of the 51- and 48-kDa FLi-1 isoforms are indicated to the right. (C and D) Graphical representations of the quantitative analysis of the production of the 51-kDa and the 48-kDa isoforms from uncapped or capped versions of the same mRNA. Results are expressed as the percentage of the amounts of either 51- or 48-kDa isoforms produced by the indicated mRNA compared to the wild type (means and standard deviations of three different experiments).