Translon: a single term for translated regions.
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Letter↗
Biomedical subjects
Petr V Sergiev
Publications and source records attributed to Petr V Sergiev.
Methyltransferase that modifies guanine 966 of the 16 S rRNA: functional identification and tertiary structure.
N(2)-Methylguanine 966 is located in the loop of Escherichia coli 16 S rRNA helix 31, forming a part of the P-site tRNA-binding pocket. We found yhhF to be a gene encoding for m(2)G966 specific 16 S rRNA methyltransferase. Disruption of the yhhF gene by kanamycin resistance marker leads to a loss of modification at G966. The modification could be rescued by expression of recombinant protein from the plasmid carrying the yhhF gene. Moreover, purified m(2)G966 methyltransferase, in the presence of S-adenosylomethionine (AdoMet), is able to methylate 30 S ribosomal subunits that were purified from yhhF knock-out strain in vitro. The methylation is specific for G966 base of the 16 S rRNA. The m(2)G966 methyltransferase was crystallized, and its structure has been determined and refined to 2.05A(.) The structure closely resembles RsmC rRNA methyltransferase, specific for m(2)G1207 of the 16 S rRNA. Structural comparisons and analysis of the enzyme active site suggest modes for binding AdoMet and rRNA to m(2)G966 methyltransferase. Based on the experimental data and current nomenclature the protein expressed from the yhhF gene was renamed to RsmD. A model for interaction of RsmD with ribosome has been proposed.
Escherichia coli↗
Identification of Escherichia coli m2G methyltransferases: II. The ygjO gene encodes a methyltransferase specific for G1835 of the 23 S rRNA.
Escherichia coli ribosomal RNA contains five guanosine residues methylated at N2. The ygjO gene was previously predicted to methylate 16 S rRNA residue G966 due to its high sequence homology with the protein RsmC, responsible for G1207 methylation. We have identified the target of YgjO as being m2G1835 of the 23 S rRNA and not m2G966 of the 16 S rRNA as expected. Knock-out of the ygjO gene leads to loss of modification at G1835, as revealed by reverse transcription. Moreover, the modification could be restored by in vivo complementation of the ygjO knock-out strain with a plasmid expressing ygjO. Recombinant YgjO protein is able to methylate in vitro protein-free 23 S rRNA, but not assembled 50 S subunits purified from the ygjO knock-out strain. The nucleotide m2G1835 is located in a functionally extremely important region of the ribosome, being located within intersubunit bridges of group B2. Growth competition assays reveal that the lack of the G1835 methylation causes growth retardation, especially at temperatures higher than optimal and in poor media. Based on these results we suggest that YgjO be renamed to RlmG in accordance with the accepted nomenclature for rRNA methyltransferases.
Base Sequence↗
Identification of Escherichia coli m2G methyltransferases: I. the ycbY gene encodes a methyltransferase specific for G2445 of the 23 S rRNA.
N2-methylguanosine 2445 of the 23 S rRNA is located in a cluster of modified nucleotides concentrated at the peptidyl transferase center of the ribosome. Here we describe the identification of a gene, ycbY, as encoding an enzyme responsible for methylation of G2445. Knock-out of the ycbY gene leads to loss of modification at G2445 as revealed by reverse transcription. The modification is restored in the ycbY knock-out strain if co-transformed with a plasmid expressing the ycbY gene product. Recombinant YcbY protein is able to methylate 23 S rRNA purified from the ycbY knock-out strain in vitro, assembled 50 S subunits are not a substrate for the methylase. Knock-out of the ycbY gene leads to growth retardation. Growth competition with the parental wild-type strain leads to a gradual decrease in the knock-out strain cells proportion in the media. It is likely that the G2445 modification is necessary for prevention of non-functional secondary or tertiary structure formation at the peptidyl transferase center. Based on these results we suggest that YcbY be renamed to RlmL in accordance with the accepted nomenclature for rRNA methyltransferases.
Base Sequence↗
Structural and functional analysis of 5S rRNA in Saccharomyces cerevisiae.
5S rRNA extends from the central protuberance of the large ribosomal subunit, through the A-site finger, and down to the GTPase-associated center. Here, we present a structure-function analysis of seven 5S rRNA alleles which are sufficient for viability in the yeast Saccharomyces cerevisiae when expressed in the absence of wild-type 5S rRNAs, and extend this analysis using a large bank of mutant alleles that show semi-dominant phenotypes in the presence of wild-type 5S rRNA. This analysis supports the hypothesis that 5S rRNA serves to link together several different functional centers of the ribosome. Data are also presented which suggest that in eukaryotic genomes selection has favored the maintenance of multiple alleles of 5S rRNA, and that these may provide cells with a mechanism to post-transcriptionally regulate gene expression.
Alleles↗
Function of the ribosomal E-site: a mutagenesis study.
Ribosomes synthesize proteins according to the information encoded in mRNA. During this process, both the incoming amino acid and the nascent peptide are bound to tRNA molecules. Three binding sites for tRNA in the ribosome are known: the A-site for aminoacyl-tRNA, the P-site for peptidyl-tRNA and the E-site for the deacylated tRNA leaving the ribosome. Here, we present a study of Escherichia coli ribosomes with the E-site binding destabilized by mutation C2394G of the 23S rRNA. Expression of the mutant 23S rRNA in vivo caused increased frameshifting and stop codon readthrough. The progression of these ribosomes through the ribosomal elongation cycle in vitro reveals ejection of deacylated tRNA during the translocation step or shortly after. E-site compromised ribosomes can undergo translocation, although in some cases it is less efficient and results in a frameshift. The mutation affects formation of the P/E hybrid site and leads to a loss of stimulation of the multiple turnover GTPase activity of EF-G by deacylated tRNA bound to the ribosome.
Binding Sites↗
The conserved A-site finger of the 23S rRNA: just one of the intersubunit bridges or a part of the allosteric communication pathway?
During the translocation of tRNAs and mRNA relative to the ribosome, the B1a, B1b and B1c bridges undergo the most extensive conformational changes among the bridges between the large and the small ribosomal subunits. The B1a bridge, also called the "A-site finger" (ASF), is formed by the 23S rRNA helix 38, which is located right above the ribosomal A-site. Here, we deleted part of the ASF so that the B1a intersubunit bridge could not be formed (DeltaB1a). The mutation led to a less efficient subunit association. A number of functional activities of the DeltaB1a ribosomes, such as tRNA binding to the P and A-sites, translocation and EF-G-related GTPase reaction were preserved. A moderate decrease in EF-G-related GTPase stimulation by the P-site occupation by deacylated tRNA was observed. This suggests that the B1a bridge is not involved in the most basic steps of the elongation cycle, but rather in the fine-tuning of the ribosomal activity. Chemical probing of ribosomes carrying the ASF truncation revealed structural differences in the 5S rRNA and in the 23S rRNA helices located between the peptidyltransferase center and the binding site of the elongation factors. Interestingly, reactivity changes were found in the P-loop, an important functional region of the 23S rRNA. It is likely that the A-site finger, in addition to its role in subunit association, forms part of the system of allosteric signal exchanges between the small subunit decoding center and the functional centers on the large subunit.
Allosteric Regulation↗
How can elongation factors EF-G and EF-Tu discriminate the functional state of the ribosome using the same binding site?
Elongation factors EF-G and EF-Tu are structural homologues and share near-identical binding sites on the ribosome, which encompass the GTPase-associated centre (GAC) and the sarcin-ricin loop (SRL). The SRL is fixed structure in the ribosome and contacts elongation factors in the vicinity of their GTP-binding site. In contrast, the GAC is mobile and we hypothesize that it interacts with the alpha helix D of the EF-Tu G-domain in the same way as with the alpha helix A of the G'-domain of EF-G. The mutual locations of these helices and GTP-binding sites in the structures of EF-Tu and EF-G are different. Thus, the orientation of the GAC relative to the SRL determines whether EF-G or EF-Tu will bind to the ribosome.
Binding Sites↗
Alteration in location of a conserved GTPase-associated center of the ribosome induced by mutagenesis influences the structure of peptidyltransferase center and activity of elongation factor G.
Translocation catalyzed by elongation factor G occurs after the peptidyltransferase reaction on the large ribosomal subunit. Deacylated tRNA in the P-site stimulates multiple turnover GTPase activity of EF-G. We suggest that the allosteric signal from the peptidyltransferase center that activates EF-G may involve the alteration in the conformation of elongation factor binding center of the ribosome. The latter consists of the moveable GTPase-associated center and the sarcin-ricin loop that keeps its position on the ribosome during translation elongation. The position of the GTPase-associated center was altered by mutagenesis. An insertion of additional base pair at positions C1030/G1124 was lethal and affected function of EF-G, but not that of EF-Tu. Structure probing revealed a putative allosteric signal pathway connecting the P-site with the binding site of the elongation factors. The results are consistent with the different structural requirements for EF-G and EF-Tu function, where the integrity of the path between the peptidyltransferase center and both GTPase-associated center and sarcin-ricin loop is important for EF-G binding.
Binding Sites↗
Affinity purification of ribosomes with a lethal G2655C mutation in 23 S rRNA that affects the translocation.
A method for preparation of Escherichia coli ribosomes carrying lethal mutations in 23 S rRNA was developed. The method is based on the site-directed incorporation of a streptavidin binding tag into functionally neutral sites of the 23 S rRNA and subsequent affinity chromatography. It was tested with ribosomes mutated at the 23 S rRNA position 2655 (the elongation factor (EF)-G binding site). Ribosomes carrying the lethal G2655C mutation were purified and studied in vitro. It was found in particular that this mutation confers strong inhibition of the translocation process but only moderately affects GTPase activity and binding of EF-G.
Base Sequence↗
Construction of the 'minimal' SRP that interacts with the translating ribosome but not with specific membrane receptors in Escherichia coli.
Escherichia coli signal recognition particle (SRP) consists of 4.5S RNA and Ffh protein. In contrast to eukaryotes, it remains unclear whether translation arrest takes place in prokaryotic cells. To study this problem we constructed a fusion of the M domain of Ffh protein with a cleavable affinity tag. This mutant Ffh, in a complex with 4.5S RNA, can bind signal peptide at the translating ribosome but is unable to bind the membrane. This SRP-ribosome complex should accumulate in the cell if translation is arrested. To test this, the complex was purified from the cells by ultracentrifugation and affinity chromatography. The composition of the complex was analyzed and found to consist of ribosomal RNAs and proteins, the Ffh M domain and 4.5S RNA. The accumulation of this complex in the cell in significant amounts indicated that SRP-mediated translation arrest did occur in bacterial cells.
Escherichia coli↗