Consecutive low-usage leucine codons block translation only when near the 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 without apparent effect on translation of other mRNAs containing CUA codons. In contrast, nine consecutive high-usage CUG leucine codons at the same position had no apparent effect, and neither low- nor high-usage codons affected translation when inserted after codon 223 or 307. Additional experiments indicated that the strong positional effect of the low-usage codons could not be accounted for by differences in stability of the mRNAs or in stringency of selection of the correct tRNA. The positional effect could be explained if translation complexes are less stable near the beginning of a message: slow translation through low-usage codons early in the message may allow most translation complexes to dissociate before they read through.