MUTATIONS IN HELIX-34 OF ESCHERICHIA-COLI 16S RIBOSOMAL-RNA HAVE MULTIPLE EFFECTS ON RIBOSOME FUNCTION AND SYNTHESIS

Helix 34 of E. coli 16 S rRNA (1046 to 1067 and 1189 to 1211) has been proposed to participate directly in the termination of translation at UGA stop codons. We have constructed mutations in this helix in plasmid-encoded rDNA to explore the specific functional roles of the sequence UCAUCA (1199 to 1204) and a secondary structure also involving positions 1054 and 1057-1058. The rRNA mutations were analyzed for their effects on in vivo translational accuracy (stop codon readthrough and frameshifting) as well as growth rate, ribosome synthesis and incorporation into polysomes. Mutations at positions 1054, 1067, 1058, 1199 and 1200 had significant effects on translational accuracy causing non-specific readthrough of all three stop codons as well as enhanced +1 and -1 frameshifting. Mutations at 1202 and 1203, however, had no effect. The incorporation of deleterious mutant subunits into 70 S ribosomes and polysomes was severely reduced and was associated with a slower growth rate and increased synthesis of host-encoded ribosomes. These data support the proposal that helix 34 is an essential component of the decoding center of the 30 S ribosomal subunit and is not restricted in function to UGA-codon specific termination.