A uridine-rich sequence required for translation of prokaryotic mRNA

Binding of 30S ribosomal subunits to mRNA during the initiation of prokaryotic translation is known to be influenced by the initiation codon and the Shine-Dalgarno sequence. Site-directed mutagenesis of rnd, the Escherichia coli gene encoding RNase D, has now shown that a U8 sequence upstream of the Shine-Dalgarno region is also essential for expression of this mRNA. Alteration of two to five uridine residues within this sequence has no effect on mRNA levels but decreases RNase D protein and activity by as much as 95%, indicating that the U-rich sequence acts as an enhancer of translation. Moreover, mutant transcripts bind to 30S ribosomes in vitro with lower affinity than their wild-type counterparts, suggesting that the role of the U8 sequence is in the initial binding of ribosomes to the translation initiation region of the message. These data demonstrate that sequences other than those previously recognized can be essential for translation initiation.