STAB-SD - A SHINE-DALGARNO SEQUENCE IN THE 5'-UNTRANSLATED REGION IS A DETERMINANT OF MESSENGER-RNA STABILITY

Transcription of the Bacillus thuringiensis cryIIIA toxin gene is init iated 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 posit ion -129 acts as a 5' mRNA stabilizer. Deletion and mutation analysis suggest that the determinant of stability is a Shine-Dalgarno (SD) seq uence close to the 5' end of the stable transcript. This SD sequence, designated STAB-SD, does not direct translation initiation. However, m utations predicted to weaken the putative interaction between the SD s equence 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 regio ns of other cryIII genes, and in the 5' untranslated regions of genes from Gram-positive bacteria other than B. thuringiensis and may theref ore be a widespread determinant of mRNA stability.