TOP mRNAs are translationally inhibited by a titratable repressor in both wheat germ extract and reticulocyte lysate

Vertebrate TOP mRNAs contain a 5' terminal oligopyrimidine tract (5' TOP), which is subject to selective translational repression in non-growing cells or in cell-free translation systems. In the present study, we monitored in vitro the effect of increasing amounts of a 16 nucleotides long oligoribon ucleotide representing the 5' terminus of moose ribosomal protein S16 mRNA on the translation of TOP and non-TOP mRNAs, Our results demonstrate that t he mild-type sequence (but not its mutant counterparts) derepresses the tra nslation of mRNAs containing 5' TOP motifs, but failed to stimulate the tra nslation of non-TOP mRNAs, even if the latter differed only by a single nuc leotide from their 5' TOP-containing counterparts, Similar results have bee n obtained with both wheat germ extract and rabbit reticulocyte lysate. It appears, therefore, that translational repression of TOP mRNAs is achieved in vitro by the accumulation of a titratable repressor rather than by the l oss of an activator and that this repressor recognizes multiple TOP mRNAs,v ith a diverse set of 5' TOP motifs. (C) 1999 Federation of European Biochem ical Societies.