STAB-SD: a Shine-Dalgarno sequence in the 5' untranslated region is a determinant of mRNA stability

Transcription of the Bacillus thuringiensis cryIIIA toxin gene is initiated at nucleotide position -558 (relative to the cryIIIA start codon). However, the major cryIIIA transcript is an mRNA with its 5' end at nucleotide position -129. Fusions to the lacZ reporter gene revealed that the cryIIIA 5' untranslated region downstream of nucleotide position -129 acts as a 5' mRNA stabilizer. Deletion and mutation analysis suggest that the determinant of stability is a Shine-Dalgarno (SD) sequence close to the 5' end of the stable transcript. This SD sequence, designated STAB-SD, does not direct translation initiation. However, mutations predicted to weaken the putative interaction between the SD sequence and the 3' end of 16S rRNA lead to reduced mRNA stability. The binding of a 30S subunit to STAB-SD may be required for stabilization of cryIIIA mRNA as a transcript with a 5' end at nucleotide position -129. Similar STAB-SD sequences are found in the 5' untranslated regions of other cryIII genes, and in the 5' untranslated regions of genes from Gram-positive bacteria other than B. thuringiensis and may therefore be a widespread determinant of mRNA stability.