Analysis of the function of E. coli 23S rRNA helix-loop 69 by mutagenesis.
BACKGROUND
The ribosome is a two-subunit enzyme known to exhibit structural dynamism during protein synthesis. The intersubunit bridges have been proposed to play important roles in decoding, translocation, and the peptidyl transferase reaction; yet the physical nature of their contributions is ill understood. An intriguing intersubunit bridge, B2a, which contains 23S rRNA helix 69 as a major component, has been implicated by proximity in a number of catalytically important regions. In addition to contacting the small ribosomal subunit, helix 69 contacts both the A and P site tRNAs and several translation factors.


RESULTS
We scanned the loop of helix 69 by mutagenesis and analyzed the mutant ribosomes using a plasmid-borne IPTG-inducible expression system. We assayed the effects of 23S rRNA mutations on cell growth, contribution of mutant ribosomes to cellular polysome pools and the ability of mutant ribosomes to function in cell-free translation. Mutations A1912G, and A1919G have very strong growth phenotypes, are inactive during in vitro protein synthesis, and under-represented in the polysomes. Mutation Psi1917C has a very strong growth phenotype and leads to a general depletion of the cellular polysome pool. Mutation A1916G, having a modest growth phenotype, is apparently defective in the assembly of the 70S ribosome.


CONCLUSIONS
Mutations A1912G, A1919G, and Psi1917C of 23S rRNA strongly inhibit translation. Mutation A1916G causes a defect in the 50S subunit or 70S formation. Mutations Psi1911C, A1913G, C1914A, Psi1915C, and A1918G lack clear phenotypes.
Liiv
Aivar
A
Estonian Biocentre, Riia 23, 51010 Tartu, Estonia. aliiv@ebc.ee
Karitkina
Diana
D
Maiväli
Ulo
U
Remme
Jaanus
J
D049109:Cell Proliferation; D002474:Cell-Free System; D004926:Escherichia coli / Q000166:cytology / Q000235:genetics*; D016296:Mutagenesis; D009154:Mutation / Q000502:physiology*; D009690:Nucleic Acid Conformation; D010641:Phenotype; D011132:Polyribosomes; D014176:Protein Biosynthesis; D012338:RNA, Ribosomal, 23S / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D012270:Ribosomes / Q000235:genetics / Q000502:physiology
D012338:RNA, Ribosomal, 23S