CONSECUTIVE LOW-USAGE LEUCINE CODONS BLOCK TRANSLATION ONLY WHEN NEARTHE 5' END OF A MESSAGE IN ESCHERICHIA-COLI

Insertion of nine consecutive low-usage CUA leucine codons after codon 13 of a 313-codon test mRNA strongly inhibited its translation withou t apparent effect on translation of other mRNAs containing CUA codons. In contrast, nine consecutive high-usage CUG leucine codons at the sa me position had no apparent effect, and neither low- nor high-usage co dons affected translation when inserted after codon 223 or 307. Additi onal experiments indicated that the strong positional effect of the lo w-usage codons could not be accounted for by differences in stability of the mRNAs or in stringency of selection of the correct tRNA. The po sitional effect could be explained if translation complexes are less s table near the beginning of a message: slow translation through low-us age codons early in the message may allow most translation complexes t o dissociate before they read through.