Substrate specificity of the pseudouridine synthase RluD in Escherichia coli

Loss of Conserved rRNA Modifications in the Peptidyl Transferase Center Leads to Diminished Protein Synthesis and Cell Growth in Budding Yeast

Ribosomal RNAs (rRNAs) are extensively modified during the transcription and subsequent maturation. Three types of modifications, 2'-O-methylation of ribose moiety, pseudouridylation, and base modifications, are introduced either by a snoRNA-driven mechanism or by stand-alone enzymes. Modified nucleotides are clustered at the functionally important sites, including peptidyl transferase center (PTC). Therefore, it has been hypothesised that the modified nucleotides play an important role in ensuring the functionality of the ribosome. In this study, we demonstrate that seven 25S rRNA modifications, including four evolutionarily conserved modifications, in the proximity of PTC can be simultaneously depleted without loss of cell viability. Yeast mutants lacking three snoRNA genes (snR34, snR52, and snR65) and/or expressing enzymatically inactive variants of spb1(D52A/E679K) and nop2(C424A/C478A) were constructed. The results show that rRNA modifications in PTC contribute collectively to efficient translation in eukaryotic cells. The deficiency of seven modified nucleotides in 25S rRNA resulted in reduced cell growth, cold sensitivity, decreased translation levels, and hyperaccurate translation, as indicated by the reduced missense and nonsense suppression. The modification m5C2870 is crucial in the absence of the other six modified nucleotides. Thus, the pattern of rRNA-modified nucleotides around the PTC is essential for optimal ribosomal translational activity and translational fidelity.

RNA Post-transcriptional Modifications of an Early-Stage Large-Subunit Ribosomal Intermediate

Protein production by ribosomes is fundamental to life, and proper assembly of the ribosome is required for protein production. The RNA, which is post-transcriptionally modified, provides the platform for ribosome assembly. Thus, a complete understanding of ribosome assembly requires the determination of the RNA post-transcriptional modifications in all of the ribosome assembly intermediates and on each pathway. There are 26 RNA post-transcriptional modifications in 23S RNA of the mature Escherichia coli (E. coli) large ribosomal subunit. The levels of these modifications have been investigated extensively only for a small number of large subunit intermediates and under a limited number of cellular and environmental conditions. In this study, we determined the level of incorporations of 2-methyl adenosine, 3-methyl pseudouridine, 5-hydroxycytosine, and seven pseudouridines in an early-stage E. coli large-subunit assembly intermediate with a sedimentation coefficient of 27S. The 27S intermediate is one of three large subunit intermediates accumulated in E. coli cells lacking the DEAD-box RNA helicase DbpA and expressing the helicase inactive R331A DbpA construct. The majority of the investigated modifications are incorporated into the 27S large subunit intermediate to similar levels to those in the mature 50S large subunit, indicating that these early modifications or the enzymes that incorporate them play important roles in the initial events of large subunit ribosome assembly.

The Fate of Duplicated Enzymes in Prokaryotes: The Case of Isomerases

The isomerases are a unique enzymatic class of enzymes that carry out a great diversity of chemical reactions at the intramolecular level. This class comprises about 300 members, most of which are involved in carbohydrate and terpenoid/polyketide metabolism. Along with oxidoreductases and translocases, isomerases are one of the classes with the highest ratio of paralogous enzymes. Due to its relatively small number of members, it is plausible to explore it in greater detail to identify specific cases of gene duplication. Here, we present an analysis at the level of individual isomerases and identify different members that seem to be involved in duplication events in prokaryotes. As was suggested in a previous study, there is no homogeneous distribution of paralogs, but rather they accumulate into a few subcategories, some of which differ between Archaea and Bacteria. As expected, the metabolic processes with more paralogous isomerases have to do with carbohydrate metabolism but also with RNA modification (a particular case involving an rRNA-modifying isomerase is thoroughly discussed and analyzed in detail). Overall, our findings suggest that the most common fate for paralogous enzymes is the retention of the original enzymatic function, either associated with a dosage effect or with differential expression in response to changing environments, followed by subfunctionalization and, to a much lesser degree, neofunctionalization, which is consistent with what has been reported elsewhere.

RNA Post-Transcriptional Modifications in Two Large Subunit Intermediates Populated in E. coli Cells Expressing Helicase Inactive R331A DbpA

23S ribosomal RNA (rRNA) of Escherichia coli 50S large ribosome subunit contains 26 post-transcriptionally modified nucleosides. Here, we determine the extent of modifications in the 35S and 45S large subunit intermediates, accumulating in cells expressing the helicase inactive DbpA protein, R331A, and the native 50S large subunit. The modifications we characterized are 3-methylpseudouridine, 2-methyladenine, 5-hydroxycytidine, and nine pseudouridines. These modifications were detected using 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide metho-p-toluenesulfonate (CMCT) treatment followed by alkaline treatment. In addition, KMnO₄ treatment of 23S rRNA was employed to detect 5-hydroxycytidine modification. CMCT and KMnO₄ treatments produce chemical changes in modified nucleotides that cause reverse transcriptase misincorporations and deletions, which were detected employing next-generation sequencing. Our results show that the 2-methyladenine modification and seven uridines to pseudouridine isomerizations are present in both the 35S and 45S to similar extents as in the 50S. Hence, the enzymes that perform these modifications, namely, RluA, RluB, RluC, RluE, RluF, and RlmN, have already acted in the intermediates. Two uridines to pseudouridine isomerizations, the 3-methylpseudouridine and 5-hydroxycytidine modifications, are significantly less present in the 35S and 45S, as compared to the 50S. Therefore, the enzymes that incorporate these modifications, RluD, RlmH, and RlhA, are in the process of modifying the 35S and 45S or will incorporate these modifications during the later stages of ribosome assembly. Our study employs a novel high throughput and single nucleotide resolution technique for the detection of 2-methyladenine and two novel high throughput and single nucleotide resolution techniques for the detection of 5-hydroxycytidine.

Pseudouridines of tRNA Anticodon Stem-Loop Have Unexpected Role in Mutagenesis in Pseudomonas sp

Pseudouridines are known to be important for optimal translation. In this study we demonstrate an unexpected link between pseudouridylation of tRNA and mutation frequency in Pseudomonas species. We observed that the lack of pseudouridylation activity of pseudouridine synthases TruA or RluA elevates the mutation frequency in Pseudomonas putida 3 to 5-fold. The absence of TruA but not RluA elevates mutation frequency also in Pseudomonas aeruginosa. Based on the results of genetic studies and analysis of proteome data, the mutagenic effect of the pseudouridylation deficiency cannot be ascribed to the involvement of error-prone DNA polymerases or malfunctioning of DNA repair pathways. In addition, although the deficiency in TruA-dependent pseudouridylation made P. putida cells more sensitive to antimicrobial compounds that may cause oxidative stress and DNA damage, cultivation of bacteria in the presence of reactive oxygen species (ROS)-scavenging compounds did not eliminate the mutator phenotype. Thus, the elevated mutation frequency in the absence of tRNA pseudouridylation could be the result of a more specific response or, alternatively, of a cumulative effect of several small effects disturbing distinct cellular functions, which remain undetected when studied independently. This work suggests that pseudouridines link the translation machinery to mutation frequency.

Variants in PUS7 Cause Intellectual Disability with Speech Delay, Microcephaly, Short Stature, and Aggressive Behavior

We describe six persons from three families with three homozygous protein truncating variants in PUS7: c.89_90del (p.Thr30Lysfs^∗20), c.1348C>T (p.Arg450^∗), and a deletion of the penultimate exon 15. All these individuals have intellectual disability with speech delay, short stature, microcephaly, and aggressive behavior. PUS7 encodes the RNA-independent pseudouridylate synthase 7. Pseudouridylation is the most abundant post-transcriptional modification in RNA, which is primarily thought to stabilize secondary structures of RNA. We show that the disease-related variants lead to abolishment of PUS7 activity on both tRNA and mRNA substrates. Moreover, pus7 knockout in Drosophila melanogaster results in a number of behavioral defects, including increased activity, disorientation, and aggressiveness supporting that neurological defects are caused by PUS7 variants. Our findings demonstrate that RNA pseudouridylation by PUS7 is essential for proper neuronal development and function.

Random pseuoduridylation in vivo reveals critical region of Escherichia coli 23S rRNA for ribosome assembly

Pseudouridine is the most common modified nucleoside in RNA, which is found in stable RNA species and in eukaryotic mRNAs. Functional analysis of pseudouridine is complicated by marginal effect of its absence. We demonstrate that excessive pseudouridines in rRNA inhibit ribosome assembly. Ten-fold increase of pseudouridines in the 16S and 23S rRNA made by a chimeric pseudouridine synthase leads to accumulation of the incompletely assembled large ribosome subunits. Hyper modified 23S rRNA is found in the r-protein assembly defective particles and are selected against in the 70S and polysome fractions showing modification interference. Eighteen positions of 23S rRNA were identified where isomerization of uridines interferes with ribosome assembly. Most of the interference sites are located in the conserved core of the large subunit, in the domain 0 of 23S rRNA, around the peptide exit tunnel. A plausible reason for pseudouridine-dependent inhibition of ribosome assembly is stabilization of rRNA structure, which leads to the folding traps of rRNA and to the retardation of the ribosome assembly.

Modulation of conformational changes in helix 69 mutants by pseudouridine modifications

Centrally located at the ribosomal subunit interface and mRNA tunnel, helix 69 (H69) from 23S rRNA participates in key steps of translation. Ribosome activity is influenced by three pseudouridine modifications, which modulate the structure and conformational behavior of H69. To understand how H69 is affected by the presence of pseudouridine in combination with sequence changes, the biophysical properties of wild-type H69 and representative mutants (A1912G, U1917C, and A1919G) were examined. Results from NMR and circular dichroism spectroscopy indicate that pH-dependent structural changes of wild-type H69 and the chosen mutants are modulated by pseudouridine and loop sequence. The effects of the mutations on global stability of H69 are negligible; however, pseudouridine stabilizes H69 at low pH conditions. Alterations to induced conformational changes of H69 likely result in compromised function, as indicated by previous biological studies.

Quantitative analysis of rRNA modifications using stable isotope labeling and mass spectrometry

Post-transcriptional RNA modifications that are introduced during the multistep ribosome biogenesis process are essential for protein synthesis. The current lack of a comprehensive method for a fast quantitative analysis of rRNA modifications significantly limits our understanding of how individual modification steps are coordinated during biogenesis inside the cell. Here, an LC-MS approach has been developed and successfully applied for quantitative monitoring of 29 out of 36 modified residues in the 16S and 23S rRNA from Escherichia coli. An isotope labeling strategy is described for efficient identification of ribose and base methylations, and a novel metabolic labeling approach is presented to allow identification of MS-silent pseudouridine modifications. The method was used to measure relative abundances of modified residues in incomplete ribosomal subunits compared to a mature ¹⁵N-labeled rRNA standard, and a number of modifications in both 16S and 23S rRNA were present in substoichiometric amounts in the preribosomal particles. The RNA modification levels correlate well with previously obtained profiles for the ribosomal proteins, suggesting that RNA is modified in a schedule comparable to the association of the ribosomal proteins. Importantly, this study establishes an efficient workflow for a global monitoring of ribosomal modifications that will contribute to a better understanding of mechanisms of RNA modifications and their impact on intracellular processes in the future.

Structure modulation of helix 69 from Escherichia coli 23S ribosomal RNA by pseudouridylations

Helix 69 (H69) is a 19-nt stem-loop region from the large subunit ribosomal RNA. Three pseudouridine (Ψ) modifications clustered in H69 are conserved across phylogeny and known to affect ribosome function. To explore the effects of Ψ on the conformations of Escherichia coli H69 in solution, nuclear magnetic resonance spectroscopy was used to reveal the structural differences between H69 with (ΨΨΨ) and without (UUU) Ψ modifications. Comparison of the two structures shows that H69 ΨΨΨ has the following unique features: (i) the loop region is closed by a Watson-Crick base pair between Ψ1911 and A1919, which is potentially reinforced by interactions involving Ψ1911N1H and (ii) Ψ modifications at loop residues 1915 and 1917 promote base stacking from Ψ1915 to A1918. In contrast, the H69 UUU loop region, which lacks Ψ modifications, is less organized. Structure modulation by Ψ leads to alteration in conformational behavior of the 5' half of the H69 loop region, observed as broadening of C1914 non-exchangeable base proton resonances in the H69 ΨΨΨ nuclear magnetic resonance spectra, and plays an important biological role in establishing the ribosomal intersubunit bridge B2a and mediating translational fidelity.

Francisella tularensis live vaccine strain folate metabolism and pseudouridine synthase gene mutants modulate macrophage caspase-1 activation

Francisella tularensis is a Gram-negative bacterium and the causative agent of the disease tularemia. Escape of F. tularensis from the phagosome into the cytosol of the macrophage triggers the activation of the AIM2 inflammasome through a mechanism that is not well understood. Activation of the AIM2 inflammasome results in autocatalytic cleavage of caspase-1, resulting in the processing and secretion of interleukin-1β (IL-1β) and IL-18, which play a crucial role in innate immune responses to F. tularensis. We have identified the 5-formyltetrahydrofolate cycloligase gene (FTL_0724) as being important for F. tularensis live vaccine strain (LVS) virulence. Infection of mice in vivo with a F. tularensis LVS FTL_0724 mutant resulted in diminished mortality compared to infection of mice with wild-type LVS. The FTL_0724 mutant also induced increased inflammasome-dependent IL-1β and IL-18 secretion and cytotoxicity in macrophages in vitro. In contrast, infection of macrophages with a F. tularensis LVS rluD pseudouridine synthase (FTL_0699) mutant resulted in diminished IL-1β and IL-18 secretion from macrophages in vitro compared to infection of macrophages with wild-type LVS. In addition, the FTL_0699 mutant was not attenuated in vivo. These findings further illustrate that F. tularensis LVS possesses numerous genes that influence its ability to activate the inflammasome, which is a key host strategy to control infection with this pathogen in vivo.

Different sensitivity of H69 modification enzymes RluD and RlmH to mutations in Escherichia coli 23S rRNA

Nucleoside modifications are introduced into the ribosomal RNA during the assembly of the ribosome. The number and the localization of the modified nucleosides in rRNAs are known for several organisms. In bacteria, rRNA modified nucleosides are synthesized by a set of specific enzymes, the majority of which have been identified in Escherichia coli. Each rRNA modification enzyme recognizes its substrate nucleoside(s) at a specific stage of ribosome assembly. Not much is known about the specificity determinants involved in the substrate recognition of the modification enzymes. In order to shed light on the substrate specificity of RluD and RlmH, the enzymes responsible for the introduction of modifications into the stem-loop 69 (H69), we monitored the formation of H69 pseudouridines (Ψ) and methylated pseudouridine (m3Ψ) in vitro on ribosomes with alterations in 23S rRNA. While the synthesis of Ψs in H69 by RluD is relatively insensitive to the point mutations at neighboring positions, methylation of one of the Ψs by RlmH exhibited a much stronger sensitivity. Apparently, in spite of synthesizing modifications in the same region or even at the same position of rRNA, the two enzymes employ different substrate recognition mechanisms.

Pseudouridylation of 23S rRNA helix 69 promotes peptide release by release factor RF2 but not by release factor RF1

Pseudouridine [Ψ] is a frequent base modification in the ribosomal RNA [rRNA] and may be involved in the modulation of the conformational flexibility of rRNA helix-loop structures during protein synthesis. Helix 69 of 23S rRNA contains pseudouridines at the positions 1911, 1915 and 1917 which are formed by the helix 69-specific synthase RluD. The growth defect caused by the lack of RluD can be rescued by mutations in class I release factor RF2, indicating a role for helix 69 pseudouridines in translation termination. We investigated the role of helix 69 pseudouridines in peptide release by release factors RF1 and RF2 in an in vitro system consisting of purified components of the Escherichia coli translation apparatus. Lack of all three pseudouridines in helix 69 compromised the activity of RF2 about 3-fold but did not significantly affect the activity of RF1. Reintroduction of pseudouridines into helix 69 by RluD-treatment restored the activity of RF2 in peptide release. A Ψ-to-C substitution at the 1917 position caused an increase in the dissociation rate of RF1 and RF2 from the postrelease ribosome. Our results indicate that the presence of all three pseudouridines in helix 69 stimulates peptide release by RF2 but has little effect on the activity of RF1. The interactions around the pseudouridine at the 1917 position appear to be most critical for a proper interaction of helix 69 with release factors.

Specificity and kinetics of 23S rRNA modification enzymes RlmH and RluD

Along the ribosome assembly pathway, various ribosomal RNA processing and modification reactions take place. Stem-loop 69 in the large subunit of Escherichia coli ribosomes plays a substantial role in ribosome functioning. It contains three highly conserved pseudouridines synthesized by pseudouridine synthase RluD. One of the pseudouridines is further methylated by RlmH. In this paper we show that RlmH has unique substrate specificity among rRNA modification enzymes. It preferentially methylates pseudouridine and less efficiently uridine. Furthermore, RlmH is the only known modification enzyme that is specific to 70S ribosomes. Kinetic parameters determined for RlmH are the following: The apparent K(M) for substrate 70S ribosomes is 0.51 ± 0.06 μM, and for cofactor S-adenosyl-L-methionine 27 ± 3 μM; the k(cat) values are 4.95 ± 1.10 min⁻¹ and 6.4 ± 1.3 min⁻¹, respectively. Knowledge of the substrate specificity and the kinetic parameters of RlmH made it possible to determine the kinetic parameters for RluD as well. The K(M) value for substrate 50S subunits is 0.98 ± 0.18 μM and the k(cat) value is 1.97 ± 0.46 min⁻¹. RluD is the first rRNA pseudouridine synthase to be kinetically characterized. The determined rates of RluD- and RlmH-directed modifications of 23S rRNA are compatible with the rate of 50S assembly in vivo. The fact that RlmH requires 30S subunits demonstrates the dependence of 50S subunit maturation on the simultaneous presence of 30S subunits.

Subribosomal particle analysis reveals the stages of bacterial ribosome assembly at which rRNA nucleotides are modified

Modified nucleosides of ribosomal RNA are synthesized during ribosome assembly. In bacteria, each modification is made by a specialized enzyme. In vitro studies have shown that some enzymes need the presence of ribosomal proteins while other enzymes can modify only protein-free rRNA. We have analyzed the addition of modified nucleosides to rRNA during ribosome assembly. Accumulation of incompletely assembled ribosomal particles (25S, 35S, and 45S) was induced by chloramphenicol or erythromycin in an exponentially growing Escherichia coli culture. Incompletely assembled ribosomal particles were isolated from drug-treated and free 30S and 50S subunits and mature 70S ribosomes from untreated cells. Nucleosides of 16S and 23S rRNA were prepared and analyzed by reverse-phase, high-performance liquid chromatography (HPLC). Pseudouridines were identified by the chemical modification/primer extension method. Based on the results, the rRNA modifications were divided into three major groups: early, intermediate, and late assembly specific modifications. Seven out of 11 modified nucleosides of 16S rRNA were late assembly specific. In contrast, 16 out of 25 modified nucleosides of 23S rRNA were made during early steps of ribosome assembly. Free subunits of exponentially growing bacteria contain undermodified rRNA, indicating that a specific set of modifications is synthesized during very late steps of ribosome subunit assembly.

YbeA is the m3Psi methyltransferase RlmH that targets nucleotide 1915 in 23S rRNA

Pseudouridines in the stable RNAs of Bacteria are seldom subjected to further modification. There are 11 pseudouridine (Psi) sites in Escherichia coli rRNA, and further modification is found only at Psi1915 in 23S rRNA, where the N-3 position of the base becomes methylated. Here, we report the identity of the E. coli methyltransferase that specifically catalyzes methyl group addition to form m(3)Psi1915. Analyses of E. coli rRNAs using MALDI mass spectrometry showed that inactivation of the ybeA gene leads to loss of methylation at nucleotide Psi1915. Methylation is restored by complementing the knockout strain with a plasmid-encoded copy of ybeA. Homologs of the ybeA gene, and thus presumably the ensuing methylation at nucleotide m(3)Psi1915, are present in most bacterial lineages but are essentially absent in the Archaea and Eukaryota. Loss of ybeA function in E. coli causes a slight slowing of the growth rate. Phylogenetically, ybeA and its homologs are grouped with other putative S-adenosylmethionine-dependent, SPOUT methyltransferase genes in the Cluster of Orthologous Genes COG1576; ybeA is the first member to be functionally characterized. The YbeA methyltransferase is active as a homodimer and docks comfortably into the ribosomal A site without encroaching into the P site. YbeA makes extensive interface contacts with both the 30S and 50S subunits to align its active site cofactor adjacent to nucleotide Psi1915. Methylation by YbeA (redesignated RlmH for rRNA large subunit methyltransferase H) possibly functions as a stamp of approval signifying that the 50S subunit has engaged in translational initiation.

Identification of pseudouridine methyltransferase in Escherichia coli

In ribosomal RNA, modified nucleosides are found in functionally important regions, but their function is obscure. Stem-loop 69 of Escherichia coli 23S rRNA contains three modified nucleosides: pseudouridines at positions 1911 and 1917, and N3 methyl-pseudouridine (m(3)Psi) at position 1915. The gene for pseudouridine methyltransferase was previously not known. We identified E. coli protein YbeA as the methyltransferase methylating Psi1915 in 23S rRNA. The E. coli ybeA gene deletion strain lacks the N3 methylation at position 1915 of 23S rRNA as revealed by primer extension and nucleoside analysis by HPLC. Methylation at position 1915 is restored in the ybeA deletion strain when recombinant YbeA protein is expressed from a plasmid. In addition, we show that purified YbeA protein is able to methylate pseudouridine in vitro using 70S ribosomes but not 50S subunits from the ybeA deletion strain as substrate. Pseudouridine is the preferred substrate as revealed by the inability of YbeA to methylate uridine at position 1915. This shows that YbeA is acting at the final stage during ribosome assembly, probably during translation initiation. Hereby, we propose to rename the YbeA protein to RlmH according to uniform nomenclature of RNA methyltransferases. RlmH belongs to the SPOUT superfamily of methyltransferases. RlmH was found to be well conserved in bacteria, and the gene is present in plant and in several archaeal genomes. RlmH is the first pseudouridine specific methyltransferase identified so far and is likely to be the only one existing in bacteria, as m(3)Psi1915 is the only methylated pseudouridine in bacteria described to date.

YccW is the m5C methyltransferase specific for 23S rRNA nucleotide 1962

Methylation at the 5-position of cytosine [m(5)C (5-methylcytidine)] occurs at three RNA nucleotides in Escherichia coli. All these modifications are at highly conserved nucleotides in the rRNAs, and each is catalyzed by its own m(5)C methyltransferase enzyme. Two of the enzymes, RsmB and RsmF, are already known and methylate 16S rRNA at nucleotides C967 and C1407, respectively. Here, we report the identity of the third E. coli m(5)C methyltransferase. Analysis of rRNAs by matrix-assisted laser desorption/ionization mass spectrometry showed that inactivation of the yccW gene leads to loss of m(5)C methylation at nucleotide 1962 in E. coli 23S rRNA. This methylation is restored by complementing the knockout strain with a plasmid-encoded copy of the yccW gene. Purified recombinant YccW protein retains its specificity for C1962 in vitro and methylates naked 23S rRNA isolated from the yccW knockout strain. However, YccW does not methylate assembled 50S subunits, and this is somewhat surprising as the published crystal structures show nucleotide C1962 to be fully accessible at the subunit interface. YccW-directed methylation at nucleotide C1962 is conserved in bacteria, and loss of this methylation in E. coli marginally reduces its growth rate. YccW had previously eluded identification because it displays only limited sequence similarity to the m(5)C methyltransferases RsmB and RsmF and is in fact more similar to known m(5)U (5-methyluridine) RNA methyltransferases. In keeping with the previously proposed nomenclature system for bacterial rRNA methyltransferases, yccW is now designated as the rRNA large subunit methyltransferase gene rlmI.

Ribosome assembly in Escherichia coli strains lacking the RNA helicase DeaD/CsdA or DbpA

Ribosome subunit assembly in bacteria is a fast and efficient process. Among the nonribosomal proteins involved in ribosome biogenesis are RNA helicases. We describe ribosome biogenesis in Escherichia coli strains lacking RNA helicase DeaD (CsdA) or DbpA. Ribosome large subunit assembly intermediate particles (40S) accumulate at 25 degrees C and at 37 degrees C in the absence of DeaD but not without DbpA. 23S rRNA is incompletely processed in the 40S and 50S particles of the DeaD(-) strain. Pulse labeling showed that the 40S particles are converted nearly completely into functional ribosomes. The rate of large ribosomal subunit assembly was reduced about four times in DeaD-deficient cells. Functional activity tests of the ribosomal particles demonstrated that the final step of 50S assembly, the activation step, was affected when DeaD was not present. The results are compatible with the model that predicts multiple DeaD-catalyzed structural transitions of the ribosome large subunit assembly.