Translation initiation complex formation with 30 S ribosomal particles mutated at conserved positions in the 3'-minor domain of 16 S RNA

Crystal structure of the Legionella pneumophila Lpg2936 in complex with the cofactor S-adenosyl-L-methionine reveals novel insights into the mechanism of RsmE family methyltransferases

The methylation of U1498 located in the 16S ribosomal RNA of Escherichia coli is an important modification affecting ribosomal activity. RsmE methyltransferases methylate specifically this position in a mechanism that requires an S‐adenosyl‐L‐methionine (AdoMet) molecule as cofactor. Here we report the structure of Apo and AdoMet‐bound Lpg2936 from Legionella pneumophila at 1.5 and 2.3 Å, respectively. The protein comprises an N‐terminal PUA domain and a C‐terminal SPOUT domain. The latter is responsible for protein dimerization and cofactor binding. Comparison with similar structures suggests that Lpg2936 is an RsmE‐like enzyme that can target the equivalent of U1498 in the L. pneumophila ribosomal RNA, thereby potentially enhancing ribosomal activity during infection‐mediated effector production. The multiple copies of the enzyme found in both structures reveal a flexible conformation of the bound AdoMet ligand. Isothermal titration calorimetry measurements suggest an asymmetric two site binding mode. Our results therefore also provide unprecedented insights into AdoMet/RsmE interaction, furthering our understanding of the RsmE catalytic mechanism.

PDB Code(s): 5O95; 5O96

The structure of Rv2372c identifies an RsmE-like methyltransferase fromMycobacterium tuberculosis

U1498 of 16S rRNA plays an important role in translation fidelity as well as in antibiotic response. U1498 is present in a methylated form in the decoding centre of the ribosome. In this study, Rv2372c fromMycobacterium tuberculosishas been identified as an RsmE-like methyltransferase which specifically methylates U1498 of 16S rRNA at the N3 position and can complement RsmE-deletedEscherichia coli. The crystal structure of Rv2372c has been determined, and reveals that the protein belongs to a distinct class in the SPOUT superfamily and exists as a dimer. The deletion of critical residues at the C-terminus of Rv2372c leads to an inability of the protein to form stable dimers and to abolition of the methyltransferase activity. A ternary model of Rv2372c with its cofactorS-adenosylmethionine (SAM) and the 16S rRNA fragment148716S rRNA1510helps to identify binding pockets for SAM (in the deep trefoil knot) and substrate RNA (at the dimer interface) and suggests an SN2 mechanism for the methylation of N3 of U1498 in 16S rRNA.

Impact of methylations of m2G966/m5C967 in 16S rRNA on bacterial fitness and translation initiation

The functional centers of the ribosome in all organisms contain ribosomal RNA (rRNA) modifications, which are introduced by specialized enzymes and come at an energy cost for the cell. Surprisingly, none of the modifications tested so far was essential for growth and hence the functional role of modifications is largely unknown. Here, we show that the methyl groups of nucleosides m²G966 and m⁵C967 of 16S rRNA in Escherichia coli are important for bacterial fitness. In vitro analysis of all phases of translation suggests that the m²G966/m⁵C967 modifications are dispensable for elongation, termination and ribosome recycling. Rather, the modifications modulate the early stages of initiation by stabilizing the binding of fMet-tRNA^fMet to the 30S pre-initiation complex prior to start-codon recognition. We propose that the m²G966 and m⁵C967 modifications help shaping the bacterial proteome, most likely by fine-tuning the rates that determine the fate of a given messenger RNA (mRNA) at early checkpoints of mRNA selection.

Fine-tuning of the ribosomal decoding center by conserved methyl-modifications in the Escherichia coli 16S rRNA

In bacterial 16S rRNAs, methylated nucleosides are clustered within the decoding center, and these nucleoside modifications are thought to modulate translational fidelity. The N⁴, 2′-O-dimethylcytidine (m⁴Cm) at position 1402 of the Escherichia coli 16S rRNA directly interacts with the P-site codon of the mRNA. The biogenesis and function of this modification remain unclear. We have identified two previously uncharacterized genes in E. coli that are required for m⁴Cm formation. mraW (renamed rsmH) and yraL (renamed rsmI) encode methyltransferases responsible for the N⁴ and 2′-O-methylations of C1402, respectively. Recombinant RsmH and RsmI proteins employed the 30S subunit (not the 16S rRNA) as a substrate to reconstitute m⁴Cm1402 in the presence of S-adenosylmethionine (Ado-Met) as the methyl donor, suggesting that m⁴Cm1402 is formed at a late step during 30S assembly in the cell. A luciferase reporter assay indicated that the lack of N⁴ methylation of C1402 increased the efficiency of non-AUG initiation and decreased the rate of UGA read-through. These results suggest that m⁴Cm1402 plays a role in fine-tuning the shape and function of the P-site, thus increasing decoding fidelity.

Identification and characterization of RsmE, the founding member of a new RNA base methyltransferase family

A variety of RNA methyltransferases act during ribosomal RNA maturation to modify nucleotides in a site-specific manner. However, of the 10 base-methylated nucleotides present in the small ribosomal subunit of Escherichia coli, only three enzymes responsible for modification of four bases are known. Here, we show that the protein encoded by yggJ, a member of the uncharacterized DUF558 protein family of predicted alpha/beta (trefoil) knot methyltransferases is responsible for methylation at U1498 in 16S rRNA. The gene is well-conserved across bacteria and plants, and likely performs the same function in other organisms. A yggJ deletion strain lacks the methyl group at U1498 as well as the specific methyltransferase activity. Moreover, purified recombinant YggJ specifically methylates m3U1498 in vitro. The deletion strain was unaffected in exponential growth in rich or minimal media at multiple temperatures, but it was defective when grown in competition with isogenic wild-type cells. Based on these data, we conclude that yggJ is the founding member of a family of RNA base methyltransferases, and propose that it be renamed rsmE.

Differential inhibition of 30S and 70S translation initiation complexes on leaderless mRNA by kasugamycin

In contrast to canonical mRNAs, translation of leaderless mRNA has been previously reported to continue in the presence of the antibiotic kasugamycin. Here, we have studied the effect of the antibiotic on determinants known to affect translation of leadered and leaderless mRNAs. Kasugamycin did not affect the Shine-Dalgarno (SD)-anti-SD (aSD) interaction or the function of translation initiation factor 3 (IF3). Thus, the preferential translation of leaderless mRNA in the presence of kasugamycin can neither be attributed to an expanding pool of 30S subunits with a "blocked" aSD nor to a lack of action of IF3, which has been shown to discriminate against translation initiation at 5'-terminal start codons. Using toeprinting, we observed that on leaderless mRNA 70S in contrast to 30S translation initiation complexes are comparatively resistant to the antibiotic. These results taken together with the known preference of 70S ribosomes for 5'-terminal AUGs lend support to the hypothesis that translation of leaderless mRNAs may as well proceed via an alternative initiation pathway accomplished by intact 70S ribosomes.

Crystal structures of complexes of the small ribosomal subunit with tetracycline, edeine and IF3

The small ribosomal subunit is responsible for the decoding of genetic information and plays a key role in the initiation of protein synthesis. We analyzed by X-ray crystallography the structures of three different complexes of the small ribosomal subunit of Thermus thermophilus with the A-site inhibitor tetracycline, the universal initiation inhibitor edeine and the C-terminal domain of the translation initiation factor IF3. The crystal structure analysis of the complex with tetracycline revealed the functionally important site responsible for the blockage of the A-site. Five additional tetracycline sites resolve most of the controversial biochemical data on the location of tetracycline. The interaction of edeine with the small subunit indicates its role in inhibiting initiation and shows its involvement with P-site tRNA. The location of the C-terminal domain of IF3, at the solvent side of the platform, sheds light on the formation of the initiation complex, and implies that the anti-association activity of IF3 is due to its influence on the conformational dynamics of the small ribosomal subunit.

Initiation factor 3-induced structural changes in the 30 S ribosomal subunit and in complexes containing tRNA(f)(Met) and mRNA

Initiation factor 3 (IF3) acts to switch the decoding preference of the small ribosomal subunit from elongator to initiator tRNA. The effects of IF3 on the 30 S ribosomal subunit and on the 30 S.mRNA. tRNA(f)(Met) complex were determined by UV-induced RNA crosslinking. Three intramolecular crosslinks in the 16 S rRNA (of the 14 that were monitored by gel electrophoresis) are affected by IF3. These are the crosslinks between C1402 and C1501 within the decoding region, between C967xC1400 joining the end loop of a helix of 16 S rRNA domain III and the decoding region, and between U793 and G1517 joining the 790 end loop of 16 S rRNA domain II and the end loop of the terminal helix. These changes occur even in the 30 S.IF3 complex, indicating they are not mediated through tRNA(f)(Met) or mRNA. UV-induced crosslinks occur between 16 S rRNA position C1400 and tRNA(f)(Met) position U34, in tRNA(f)(Met) the nucleotide adjacent to the 5' anticodon nucleotide, and between 16 S rRNA position C1397 and the mRNA at positions +9 and +10 (where A of the initiator AUG codon is +1). The presence of IF3 reduces both of these crosslinks by twofold and fourfold, respectively. The binding site for IF3 involves the 790 region, some other parts of the 16 S rRNA domain II and the terminal stem/loop region. These are located in the front bottom part of the platform structure in the 30 S subunit, a short distance from the decoding region. The changes that occur in the decoding region, even in the absence of mRNA and tRNA, may be induced by IF3 from a short distance or could be caused by the second IF3 structural domain.

Conformational analysis of Escherichia coli 30S ribosomes containing the single-base mutations G530U, U1498G, G1401C, and C1501G and the double-base mutation G1401C/C1501G

Biochemical and genetic studies have pointed out the importance of several sites in 16S ribosomal RNA of Escherichia coli in the decoding process. These sites consist of the core of the decoding center (1400/1500 region) and two other segments (530 and 1050/1200 regions). To detect a possible structural link between these functionally related regions, we analyzed their sensitivity to conformational changes induced by mutations which are located in each of these regions and are known to affect the decoding process. The conformations of five segments of 16S rRNA (1-106, 406-569, 780-978, 997-1247, and 1334-1519) were analyzed by chemical probing of 30S ribosomes containing the following mutations: G530U, U1498G, G1401C, C1501G, and G1401C/C1501G. Ribosomes reconstituted with natural wild-type 16S RNA showed only minor conformational differences with respect to ribosomes isolated from cells. When 16S RNA made in vitro replaced natural 16S RNA, a slightly looser conformation of the central core region was found. Mutant ribosomes made by reconstitution with mutant 16S RNA made in vitro showed conformational effects which were in all cases localized to the region of secondary structure surrounding the site of mutation. Although the core of the decoding center (1400/1500 region) and the two other sites (530 and 1050/1200 regions) participating in the decoding function have been functionally linked, our data indicate that they are structurally independent. They also provide evidence for an unusual structure of the 1400/1500 decoding center, possibly involving noncanonical interactions. Furthermore, the absence of any conformational effect induced by the G530U mutation except at the site of mutation itself points to its direct, as opposed to indirect, involvement in the decoding function of the ribosome.

Recognition of the mRNA selenocysteine insertion sequence by the specialized translational elongation factor SELB

In Escherichia coli the unusual amino acid selenocysteine is incorporated cotranslationally at an in-frame UGA codon. Incorporation of selenocysteine relies, in part, on the interaction between a specialized elongation factor, the SELB protein, and a cis-acting element within the mRNA. Boundary and toeprint experiments illustrate that the SELB-GTP-Sec-tRNA(Sec) ternary complex binds to the selenoprotein encoding mRNAs fdhF and fdnG, serving to increase the concentration of SELB and Sec-tRNA(Sec) on these mRNAs in vivo. Moreover, toeprint experiments indicate that SELB recognizes the ribosome-bound message and that, upon binding, SELB may protrude out of the ribosomal-mRNA track so as to approach the large ribosomal subunit. The results place the mRNA-bound SELB-GTP-Sec-tRNA(Sec) ternary complex at the selenocysteine codon (as expected) and suggest a mechanism to explain the specificity of selenocysteine insertion. Cis-acting mRNA regulatory elements can tether protein factors to the translation complex during protein synthesis.

Methylation of the conserved A1518-A1519 in Escherichia coli 16S ribosomal RNA by the ksgA methyltransferase is influenced by methylations around the similarly conserved U1512.G1523 base pair in the 3' terminal hairpin

An in vitro system developed for the site-specific mutagenesis of 16S rRNA of Escherichia coli ribosomes was used to make five mutations around the highly conserved U1512.G1523 base pair in the 3' terminal hairpin. Each of the mutant RNAs was reconstituted with a complete mixture of 30S proteins to yield 30S ribosomal particles, which were tested for the ability of the ksgA methylase to form m6(2)A1518 and m6(2)A1519. Dimethylation of A1518 and A1519 in the hairpin loop was inhibited 20-80% by the mutations. The results indicate that G1523 and C1524 in the stem are important determinants for the dimethylation of A1518 and A1519 in the loop. Either the enzyme recognition region extends that far or the effect of mutations in the stem are propagated in some manner to the loop. The conserved U.G base pair does not of itself appear to play a major role in ksgA methylase recognition.