MUTATIONS AFFECTING THE SHINE-DALGARNO SEQUENCES OF THE UNTRANSLATED REGION OF THE ESCHERICHIA-COLI GLTBDF OPERON

Individual mutations which affected each of the two Shine-Dalgarno sequences at the 5' untranslated region of the gltB gene of Escherichia coli were characterized. They were isolated in plasmids carrying a gltB'-'lacZ protein fusion preceded by the regulatory region of the gltBDF operon. Subcloning and nucleotide sequencing of almost-equal-to 1,206 bp of DNA encompassing the gltBDF regulatory region showed that the mutations affected the first base at each of the two identical Shine-Dalgarno sequences, SD1 and SD2, located 40 and 8 bases, respectively, upstream from the putative gltB open reading frame. Only mutation gltB2r227, an adenine in place of a guanine, affecting the first base of SD2, lowered beta-galactosidase expression significantly, i.e., about fivefold. The results suggest that SD2 is the preferred functional site at which ribosomes initiate gltB mRNA translation.