Book of Abstracts: Albany 2003
June 17-21 2003
Visualizing tmRNA Entry into the Stalled Ribosome
Bacterial ribosomes stalled on defective messenger RNAs (mRNAs) are rescued by tmRNA, a ~300 nucleotide long molecule that functions as both transfer RNA (tRNA) and mRNA. Translation then switches from the defective mRNA to a short open reading frame (ORF) on tmRNA that tags the defective nascent peptide chain for degradation. However, the mechanism by which tmRNA can enter and move through the ribosome is unknown. We present a cryo-electron microscopy study at ~13-15Å showing the entry of tmRNA from Thermus thermophilus in a complex with EF-Tu·GDP, stalled with the antibiotic kirromycin, into the ribosome. Apart from EF-Tu, the tmRNA interaction with the ribosome requires two other proteins; a small protein B (SmpB), which has been shown to be required for tmRNA-mediated peptide-tagging activity, and ribosomal protein S1, which is known to bind single-stranded RNA and has been proposed to be required for tmRNA binding to the ribosome. The cryo-EM structure gives clues on how tmRNA could move through the ribosome despite its complicated topology and addresses a number of interesting questions regarding the way tmRNA functions. It shows that EF-Tu brings aminoacylated tmRNA to the ribosome in a manner similar to canonical tRNAs. The protein SmpB bridges tmRNA and the 50S subunit. A mimicry of codon-anticodon base pairing between distal parts of tmRNA is shown to be unlikely; rather, an internal loop between helices 2a and 2b is situated close to the decoding site. Protein S1 is potentially involved in unwinding a portion of the ORF. The cryo-EM structure reveals the locations of the pseudoknots and sheds some light on their individual roles.
1Howard Hughes Medical Institute