FORMATION OF THE CENTRAL PSEUDOKNOT IN 16S RIBOSOMAL-RNA IS ESSENTIALFOR INITIATION OF TRANSLATION

The postulated central pseudoknot formed by regions 9-13/21-25 and 17- 19/916-918 of 16S rRNA of Escherichia coli is phylogenetically conserv ed in prokaryotic as well eukaryotic species. This pseudoknot is locat ed at the center of the secondary structure of the 16S rRNA and connec ts the three major domains of this molecule. We have introduced mutati ons into this pseudoknot by changing the base-paired residues C18 and G917, and the effect of such mutations on the ribosomal activity was s tudied in vivo, using a 'specialized' ribosome system. As compared wit h ribosomes having the wild-type pseudoknot, the translational activit y of ribosomes containing an A, G or U residue at position 18 was dram atically reduced, while the activity of mutant ribosomes having comple mentary bases at positions 18 and 917 was at the wild-type level. The reduced translational activity of those mutants that are incapable of forming a pseudoknot was caused by their inability to form 70S ribosom al complexes. These results demonstrate that the potential formation o f a central pseudoknot in 16S rRNA with any base-paired residues at po sitions 18 and 917 is essential to complete the initiation process.