Structures of tRNAs with an expanded anticodon loop in the decoding center of the 30S ribosomal subunit.

During translation, some +1 frameshift mRNA sites are decoded by frameshift suppressor tRNAs that contain an extra base in their anticodon loops. Similarly engineered tRNAs have been used to insert nonnatural amino acids into proteins. Here, we report crystal structures of two anticodon stem-loops (ASLs) from tRNAs known to facilitate +1 frameshifting bound to the 30S ribosomal subunit with their cognate mRNAs. ASL(CCCG) and ASL(ACCC) (5'-3' nomenclature) form unpredicted anticodon-codon interactions where the anticodon base 34 at the wobble position contacts either the fourth codon base or the third and fourth codon bases. In addition, we report the structure of ASL(ACGA) bound to the 30S ribosomal subunit with its cognate mRNA. The tRNA containing this ASL was previously shown to be unable to facilitate +1 frameshifting in competition with normal tRNAs (Hohsaka et al. 2001), and interestingly, it displays a normal anticodon-codon interaction. These structures show that the expanded anticodon loop of +1 frameshift promoting tRNAs are flexible enough to adopt conformations that allow three bases of the anticodon to span four bases of the mRNA. Therefore it appears that normal triplet pairing is not an absolute constraint of the decoding center.

The A-site finger in 23 S rRNA acts as a functional attenuator for translocation.

Helix 38 (H38) in 23 S rRNA, which is known as the "A-site finger (ASF)," is located in the intersubunit space of the ribosomal 50 S subunit and, together with protein S13 in the 30 S subunit, it forms bridge B1a. It is known that throughout the decoding process, ASF interacts directly with the A-site tRNA. Bridge B1a becomes disrupted by the ratchet-like rotation of the 30 S subunit relative to the 50 S subunit. This occurs in association with elongation factor G (EF-G)-catalyzed translocation. To further characterize the functional role(s) of ASF, variants of Escherichia coli ribosomes with a shortened ASF were constructed. The E. coli strain bearing such ASF-shortened ribosomes had a normal growth rate but enhanced +1 frameshift activity. ASF-shortened ribosomes showed normal subunit association but higher activity in poly(U)-dependent polyphenylalanine synthesis than the wild type (WT) ribosome at limited EF-G concentrations. In contrast, other ribosome variants with shortened bridge-forming helices 34 and 68 showed weak subunit association and less efficient translational activity than the WT ribosome. Thus, the higher translational activity of ASF-shortened ribosomes is caused by the disruption of bridge B1a and is not due to weakened subunit association. Single round translocation analyses clearly demonstrated that the ASF-shortened ribosomes have higher translocation activity than the WT ribosome. These observations indicate that the intrinsic translocation activity of ribosomes is greater than that usually observed in the WT ribosome and that ASF is a functional attenuator for translocation that serves to maintain the reading frame.

Translocation of a tRNA with an extended anticodon through the ribosome.

Coordinated translocation of the tRNA-mRNA complex by the ribosome occurs in a precise, stepwise movement corresponding to a distance of three nucleotides along the mRNA. Frameshift suppressor tRNAs generally contain an extra nucleotide in the anticodon loop and they subvert the normal mechanisms used by the ribosome for frame maintenance. The mechanism by which suppressor tRNAs traverse the ribosome during translocation is poorly understood. Here, we demonstrate translocation of a tRNA by four nucleotides from the A site to the P site, and from the P site to the E site. We show that translocation of a punctuated mRNA is possible with an extra, unpaired nucleotide between codons. Interestingly, the NMR structure of the four nucleotide anticodon stem-loop reveals a conformation different from the canonical tRNA structure. Flexibility within the loop may allow conformational adjustment upon A site binding and for interacting with the four nucleotide codon in order to shift the mRNA reading frame.

Non-bridging phosphate oxygen atoms within the tRNA anticodon stem-loop are essential for ribosomal A site binding and translocation.

The conformation of the anticodon stem-loop of tRNAs required for correct decoding by the ribosome depends on intramolecular and intermolecular interactions that are independent of the tRNA nucleotide sequence. Non-bridging phosphate oxygen atoms have been shown to be critical for the structure and function of several RNAs. However, little is known about the role they play in ribosomal A site binding and translocation of tRNA to the P site. Here, we show that non-bridging phosphate oxygen atoms within the tRNA anticodon stem-loop at positions 33, 35, and 37 are important for A site binding. Those at positions 34 and 36 are not necessary for binding, but are essential for translocation. Our results correlate with structural data, indicating that position 34 interacts with the highly conserved 16S rRNA base G966 and position 36 interacts with the universally conserved tRNA base U33 during translocation to the P site.

Modified nucleotides in tRNA(Lys) and tRNA(Val) are important for translocation.

Ribosomes translate genetic information encoded by messenger RNAs (mRNAs) into proteins. Accurate decoding by the ribosome depends on the proper interaction between the mRNA codon and the anticodon of transfer RNA (tRNA). tRNAs from all kingdoms of life are enzymatically modified at distinct sites, particularly in and near the anticodon. Yet, the role of these naturally occurring tRNA modifications in translation is not fully understood. Here we show that modified nucleosides at the first, or wobble, position of the anticodon and 3'-adjacent to the anticodon are important for translocation of tRNA from the ribosome's aminoacyl site (A site) to the peptidyl site (P site). Thus, naturally occurring modifications in tRNA contribute functional groups and conformational dynamics that are critical for accurate decoding of mRNA and for translocation to the P site during protein synthesis.

Universally conserved interactions between the ribosome and the anticodon stem-loop of A site tRNA important for translocation.

The iterative movement of the tRNA-mRNA complex through the ribosome is a hallmark of the elongation phase of protein synthesis. We used synthetic anticodon stem-loop analogs (ASL) of tRNA(Phe) to systematically identify ribose 2'-hydroxyl groups that are essential for binding and translocation from the ribosomal A site. Our results show that 2'-hydroxyl groups at positions 33, 35, and 36 in the A site ASL are important for translocation. Consistent with the view that the molecular basis of translocation may be similar in all organisms, the 2'-hydroxyl groups at positions 35 and 36 in the ASL interact with universally conserved bases G530 and A1493, respectively, in 16S rRNA. Furthermore, these interactions are also essential for the decoding process, indicating a functional relationship between decoding and translocation.