The Tn10-encoded tetracycline resistance mRNA contains a translational silencer in the 5' nontranslated region.

We performed a mutational analysis of the left half of Tn10-encoded tet operator O2, located in the 5' nontranslated region of the mRNA for the resistance protein TetA, and determined the importance of that region for translation efficiency and mRNA stability. Transcriptional fusions of 17 mutants to lacZ expressed the same amounts of beta-galactosidase, while translational fusions varied 35-fold in expression efficiency. The mRNA half-lives varied 24-fold, with 9.6 min for the most highly expressed mRNA and 0.4 min for the least efficiently expressed mRNA. Toeprint experiments were performed to distinguish whether these mutations define a determinant of mRNA stability or influence translation initiation. The highly expressed mRNA was 24-fold more efficient in forming the initiation complex in vitro than the low-expression mutant. It was concluded that this sequence, albeit located upstream of the ribosome-binding sequence, is an important determinant for efficient initiation of translation. Secondary-structure calculations of the mRNAs revealed no correlation of the potential to form double strands masking the ribosome-binding sequence with expression efficiency.