The authors note that on page 2572, left column, in line 16 of the second full paragraph of the Results section, the sentence, ''Based on the dye locations and previous biochemical and structural characterization of similar complexes, we infer that ribosome populations primarily in a low-FRET regime correspond to an open state of the L1 stalk with populations primarily in the high-FRET regime in a closed conformation of the L1 stalk in the P/E hybrid state,'' should instead read ''Based on the dye locations and previous biochemical and structural characterization of similar complexes, we infer that the low-FRET population corresponds to an open state of the L1 stalk and the high-FRET population to a closed conformation of the L1 stalk in the P/E hybrid state.'' The authors note that due to a printer's error, on page 2575, left column, in line 4 of the first full paragraph, the sentence, ''After the movement of the L1 stalk, we were able to enrich for populations of half-closed (Ϸ0.4 FRET) complexes by addition of excess deacylated tRNA to classical state (Ϸ0.25 FRET) complexes containing a vacant E site ( Fig. 3 C and E),'' should instead appear as ''Following the movement of the L1 stalk, we were able to enrich for populations of half-closed (Ϸ0.4 FRET) complexes by addition of excess deacylated tRNA to classical state (Ϸ0.25 FRET) complexes containing a vacant E site ( Fig. 3 C and E).'' Additionally, the authors note that in Fig. 6, the y-axis of panel B was labeled incorrectly. The corrected figure and its legend appear below. (9) and L1 stalk movement (this work; Table S1). The dashed lines at K eq ϭ 1 divide the plot into 4 quadrants corresponding to the 4 possible combinations of nonrotated and rotated orientations of the subunits and fully closed and open conformations of the L1 stalk. Filled squares correspond to complexes of pretranslocation ribosomes containing deacylated tRNA in the P site (tRNA Tyr , tRNA Phe , or tRNA fMet ). Open circles correspond to posttranslocation ribosomes containing N-Ac-Phe-tRNA Phe in the P site and a vacant E site. Open triangles correspond to vacant ribosomes with or without EF-G⅐GDPNP bound. (B) Correlation between forward rates: closing of the L1 stalk vs. rotation of subunits from classical to hybrid state. (C) Correlation between reverse rates: opening of the L1 stalk vs. rotation of subunits from hybrid to classical state. Lines represent log-linear fits of the data.

Ribosome crystal structures have revealed that two small subunit proteins, S9 and S13, have C-terminal tails, which, together with several features of 16S rRNA, contact the anticodon stem-loop of P-site tRNA. To test the functional importance of these protein tails, we created genomic deletions of the C-terminal regions of S9 and S13. All of the tail deletions, including double mutants containing deletions in both S9 and S13, were viable, showing that Escherichia coli cells can synthesize all of their proteins by using ribosomes that contain 30S P sites composed only of RNA. However, these mutants have slower growth rates, indicating that the tails may play a supporting functional role in translation. In vitro analysis shows that 30S subunits purified from the S13 deletion mutants have a generally decreased affinity for tRNA, whereas deletion of the S9 tail selectively affects the binding of tRNAs whose anticodon stem sequences are most divergent from that of initiator tRNA. E xtensive evidence from more than three decades of experiments (1) supports the idea that ribosomal function is based primarily on rRNA (2, 3). This is most clearly seen in ribosome crystal structures, which show that the functional region of the ribosome, at the subunit interface, is composed mostly of rRNA, whereas the ribosomal proteins are found mainly at the periphery of the ribosome (4-7). However, there are a number of instances where ribosomal proteins contact tRNA (6). One of these occurs in the 30S P site, which plays a major role in initiation of protein synthesis, binding the initiator tRNA and positioning the start codon of mRNA. The 30S P site is also responsible for binding and positioning the anticodon stem-loop (ASL) of peptidyl-tRNA during polypeptide elongation and for maintaining the translational reading frame when the A site is vacant. Binding of the ASL depends not only on base-pairing with the P-site codon of mRNA, but also on interactions with the 30S subunit itself. In addition to contacts involving several elements of 16S rRNA, the extended C-terminal tails of two proteins, S9 and S13, penetrate the 30S P site within contact distance of the ASL (5, 6). The universally conserved C-terminal arginine of S9 appears to contact phosphate 35 at the apex of the anticodon loop of the P-site tRNA (Fig. 1). The C-terminal tail of S9 is phylogenetically invariant in length, an observation that is explained by the crystal structure, which shows that the P-site tRNA would physically block extension of the S9 tail. The tail of S13 runs parallel to the anticodon stem, crossing the tRNA backbone between positions 29 and 30 and is thus more tolerant of variations in length (Fig. 1). Although more variable in sequence, the S13 tail always contains several basic side chains, which can make electrostatic interactions with tRNA phosphates. The interactions of the S9 and S13 protein tails with P-site tRNA and their phylogenetic conservation suggest that they are directly involved in the function of the 30S P site.Early in vitro reconsti...

The 30S ribosomal P site serves several functions in translation. It must specifically bind initiator tRNA during formation of the 30S initiation complex; bind the anticodon stemloop of peptidyl-tRNA during the elongation phase; and help to maintain the translational reading frame when the A site is unoccupied. Early experiments provided evidence that 16S rRNA was an important component of the 30S P site. Footprinting and crosslinking studies later implicated specific nucleotides in interactions with tRNA. The crystal structures of the 30S subunit and 70S ribosome-tRNA complexes confirmed the interactions between 16S rRNA and tRNA, but also revealed contacts between tRNA and the C-terminal tails of proteins S9 and S13. Deletion of these tails now shows that the 16S rRNA contacts alone are sufficient to support protein synthesis in living cells.

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