Mutational analysis defines the roles of conserved amino acid residues in the predicted catalytic pocket of the rRNA:m6A methyltransferase ErmC'

Two dynamic N-terminal regions are required for function in ribosomal RNA adenine dimethylase family members

Prominent members of the ribosomal RNA adenine dimethylase (RRAD) family of enzymes facilitate ribosome maturation by dimethylating 2 nt of small subunit rRNA, including the human DIMT1 and bacterial KsgA enzymes. A subgroup of RRAD enzymes, named erythromycin resistance methyltransferases (Erm), dimethylate a specific nucleotide in large subunit rRNA to confer antibiotic resistance. How these enzymes regulate methylation so that it only occurs on the specific substrate is not fully understood. While performing random mutagenesis on the catalytic domain of ErmE, we discovered that mutants in an N-terminal region of the protein that is disordered in the ErmE crystal structure are associated with a loss of antibiotic resistance. By subjecting site-directed mutants of ErmE and KsgA to phenotypic and in vitro assays, we found that the N-terminal region is critical for activity in RRAD enzymes: The N-terminal basic region promotes rRNA binding, and the conserved motif likely assists in juxtaposing the adenosine substrate and the S-adenosylmethionine cofactor. Our results and emerging structural data suggest that this dynamic, N-terminal region of RRAD enzymes becomes ordered upon rRNA binding, forming a cap on the active site required for methylation.

Three critical regions of the erythromycin resistance methyltransferase, ErmE, are required for function supporting a model for the interaction of Erm family enzymes with substrate rRNA

6-Methyladenosine modification of DNA and RNA is widespread throughout the three domains of life and often accomplished by a Rossmann-fold methyltransferase domain which contains conserved sequence elements directing S-adenosylmethionine cofactor binding and placement of the target adenosine residue into the active site. Elaborations to the conserved Rossman-fold and appended domains direct methylation to diverse DNA and RNA sequences and structures. Recently, the first atomic-resolution structure of a ribosomal RNA adenine dimethylase (RRAD) family member bound to rRNA was solved, TFB1M bound to helix 45 of 12S rRNA. Since erythromycin resistance methyltransferases are also members of the RRAD family, and understanding how these enzymes recognize rRNA could be used to combat their role in antibiotic resistance, we constructed a model of ErmE bound to a 23S rRNA fragment based on the TFB1M-rRNA structure. We designed site-directed mutants of ErmE based on this model and assayed the mutants by in vivo phenotypic assays and in vitro assays with purified protein. Our results and additional bioinformatic analyses suggest our structural model captures key ErmE-rRNA interactions and indicate three regions of Erm proteins play a critical role in methylation: the target adenosine binding pocket, the basic ridge, and the α4-cleft.

Potential Target Site for Inhibitors in MLSB Antibiotic Resistance

Macrolide-lincosamide-streptogramin B antibiotic resistance occurs through the action of erythromycin ribosome methylation (Erm) family proteins, causing problems due to their prevalence and high minimal inhibitory concentration, and feasibilities have been sought to develop inhibitors. Erms exhibit high conservation next to the N-terminal end region (NTER) as in ErmS, 64SQNF67. Side chains of homologous S, Q and F in ErmC' are surface-exposed, located closely together and exhibit intrinsic flexibility; these residues form a motif X. In S64 mutations, S64G, S64A and S64C exhibited 71%, 21% and 20% activity compared to the wild-type, respectively, conferring cell resistance. However, mutants harboring larger side chains did not confer resistance and retain the methylation activity in vitro. All mutants of Q65, Q65N, Q65E, Q65R, and Q65H lost their methyl group transferring activity in vivo and in vitro. At position F67, a size reduction of side-chain (F67A) or a positive charge (F67H) greatly reduced the activity to about 4% whereas F67L with a small size reduction caused a moderate loss, more than half of the activity. The increased size by F67Y and F67W reduced the activity by about 75%. In addition to stabilization of the cofactor, these amino acids could interact with substrate RNA near the methylatable adenine presumably to be catalytically well oriented with the SAM (S-adenosyl-L-methionine). These amino acids together with the NTER beside them could serve as unique potential inhibitor development sites. This region constitutes a divergent element due to the NTER which has variable length and distinct amino acids context in each Erm. The NTER or part of it plays critical roles in selective recognition of substrate RNA by Erms and this presumed target site might assume distinct local structure by induced conformational change with binding to substrate RNA and SAM, and contribute to the specific recognition of substrate RNA.

Shared requirements for key residues in the antibiotic resistance enzymes ErmC and ErmE suggest a common mode of RNA recognition

Erythromycin-resistance methyltransferases are SAM dependent Rossmann fold methyltransferases that convert A2058 of 23S rRNA to m⁶₂A2058. This modification sterically blocks binding of several classes of antibiotics to 23S rRNA, resulting in a multidrug-resistant phenotype in bacteria expressing the enzyme. ErmC is an erythromycin resistance methyltransferase found in many Gram-positive pathogens, whereas ErmE is found in the soil bacterium that biosynthesizes erythromycin. Whether ErmC and ErmE, which possess only 24% sequence identity, use similar structural elements for rRNA substrate recognition and positioning is not known. To investigate this question, we used structural data from related proteins to guide site-saturation mutagenesis of key residues and characterized selected variants by antibiotic susceptibility testing, single turnover kinetics, and RNA affinity-binding assays. We demonstrate that residues in α4, α5, and the α5-α6 linker are essential for methyltransferase function, including an aromatic residue on α4 that likely forms stacking interactions with the substrate adenosine and basic residues in α5 and the α5-α6 linker that likely mediate conformational rearrangements in the protein and cognate rRNA upon interaction. The functional studies led us to a new structural model for the ErmC or ErmE-rRNA complex.

Binding studies of a putative C. pseudotuberculosis target protein from Vitamin B12 Metabolism

Vitamin B12 acts as a cofactor for various metabolic reactions important in living organisms. The Vitamin B12 biosynthesis is restricted to prokaryotes, which means, all eukaryotic organisms must acquire this molecule through diet. This study presents the investigation of Vitamin B12 metabolism and the characterization of precorrin-4 C(11)-methyltransferase (CobM), an enzyme involved in the biosynthesis of Vitamin B12 in Corynebacterium pseudotuberculosis. The analysis of the C. pseudotuberculosis genome identified two Vitamin B12-dependent pathways, which can be strongly affected by a disrupted vitamin metabolism. Molecular dynamics, circular dichroism, and NMR-STD experiments identified regions in CobM that undergo conformational changes after s-adenosyl-L-methionine binding to promote the interaction of precorrin-4, a Vitamin B12 precursor. The binding of s-adenosyl-L-methionine was examined along with the competitive binding of adenine, dATP, and suramin. Based on fluorescence spectroscopy experiments the dissociation constant for the four ligands and the target protein could be determined; SAM (1.4 ± 0.7 µM), adenine (17.8 ± 1.5 µM), dATP (15.8 ± 2.0 µM), and Suramin (6.3 ± 1.1 µM). The results provide rich information for future investigations of potential drug targets within the C. pseudotuberculosis's Vitamin B12 metabolism and related pathways to reduce the pathogen's virulence in its hosts.

Chemistry enters nucleic acids biology: enzymatic mechanisms of RNA modification

Modified nucleotides are universally conserved in all living kingdoms and are present in almost all types of cellular RNAs, including tRNA, rRNA, sn(sno)RNA, and mRNA and in recently discovered regulatory RNAs. Altogether, over 110 chemically distinct RNA modifications have been characterized and localized in RNA by various analytical methods. However, this impressive list of known modified nucleotides is certainly incomplete, mainly due to difficulties in identification and characterization of these particular residues in low abundance cellular RNAs. In DNA, modified residues are formed by both enzymatic reactions (like DNA methylations, for example) and by spontaneous chemical reactions resulting from oxidative damage. In contrast, all modified residues characterized in cellular RNA molecules are formed by specific action of dedicated RNA-modification enzymes, which recognize their RNA substrate with high specificity. These RNA-modification enzymes display a great diversity in terms of the chemical reaction and use various low molecular weight cofactors (or co-substrates) in enzymatic catalysis. Depending on the nature of the target base and of the co-substrate, precise chemical mechanisms are used for appropriate activation of the base and the co-substrate in the enzyme active site. In this review, we give an extended summary of the enzymatic mechanisms involved in formation of different methylated nucleotides in RNA, as well as pseudouridine residues, which are almost universally conserved in all living organisms. Other interesting mechanisms include thiolation of uridine residues by ThiI and the reaction of guanine exchange catalyzed by TGT. The latter implies the reversible cleavage of the N-glycosidic bond in order to replace the initially encoded guanine by an aza-guanosine base. Despite the extensive studies of RNA modification and RNA-modification machinery during the last 20 years, our knowledge on the exact chemical steps involved in catalysis of RNA modification remains very limited. Recent discoveries of radical mechanisms involved in base methylation clearly demonstrate that numerous possibilities are used in Nature for these difficult reactions. Future studies are certainly required for better understanding of the enzymatic mechanisms of RNA modification, and this knowledge is crucial not only for basic research, but also for development of new therapeutic molecules.

Fast and sensitive alignment of microbial whole genome sequencing reads to large sequence datasets on a desktop PC: application to metagenomic datasets and pathogen identification

Next generation sequencing (NGS) of metagenomic samples is becoming a standard approach to detect individual species or pathogenic strains of microorganisms. Computer programs used in the NGS community have to balance between speed and sensitivity and as a result, species or strain level identification is often inaccurate and low abundance pathogens can sometimes be missed. We have developed Taxoner, an open source, taxon assignment pipeline that includes a fast aligner (e.g. Bowtie2) and a comprehensive DNA sequence database. We tested the program on simulated datasets as well as experimental data from Illumina, IonTorrent, and Roche 454 sequencing platforms. We found that Taxoner performs as well as, and often better than BLAST, but requires two orders of magnitude less running time meaning that it can be run on desktop or laptop computers. Taxoner is slower than the approaches that use small marker databases but is more sensitive due the comprehensive reference database. In addition, it can be easily tuned to specific applications using small tailored databases. When applied to metagenomic datasets, Taxoner can provide a functional summary of the genes mapped and can provide strain level identification. Taxoner is written in C for Linux operating systems. The code and documentation are available for research applications at http://code.google.com/p/taxoner.

Deregulation of translation due to post-transcriptional modification of rRNA explains why erm genes are inducible

A key mechanism of bacterial resistance to macrolide antibiotics is the dimethylation of a nucleotide in the large ribosomal subunit by erythromycin resistance methyltransferases. The majority of erm genes are expressed only when the antibiotic is present and the erythromycin resistance methyltransferase activity is critical for the survival of bacteria. Although these genes were among the first discovered inducible resistance genes, the molecular basis for their inducibility has remained unknown. Here we show that erythromycin resistance methyltransferase expression reduces cell fitness. Modification of the nucleotide in the ribosomal tunnel skews the cellular proteome by deregulating the expression of a set of proteins. We further demonstrate that aberrant translation of specific proteins results from abnormal interactions of the nascent peptide with the erythromycin resistance methyltransferase-modified ribosomal tunnel. Our findings provide a plausible explanation why erm genes have evolved to be inducible and underscore the importance of nascent peptide recognition by the ribosome for generating a balanced cellular proteome.

A novel hybrid SCCmec-mecC region in Staphylococcus sciuri

OBJECTIVES

Methicillin resistance in Staphylococcus spp. results from the expression of an alternative penicillin-binding protein 2a (encoded by mecA) with a low affinity for β-lactam antibiotics. Recently, a novel variant of mecA known as mecC (formerly mecALGA251) was identified in Staphylococcus aureus isolates from both humans and animals. In this study, we identified two Staphylococcus sciuri subsp. carnaticus isolates from bovine infections that harbour three different mecA homologues: mecA, mecA1 and mecC.

METHODS

We subjected the two isolates to whole-genome sequencing to further understand the genetic context of the mec-containing region. We also used PCR and RT-PCR to investigate the excision and expression of the SCCmec element and mec genes, respectively.

RESULTS

Whole-genome sequencing revealed a novel hybrid SCCmec region at the orfX locus consisting of a class E mec complex (mecI-mecR1-mecC1-blaZ) located immediately downstream of a staphylococcal cassette chromosome mec (SCCmec) type VII element. A second SCCmec attL site (attL2), which was imperfect, was present downstream of the mecC region. PCR analysis of stationary-phase cultures showed that both the SCCmec type VII element and a hybrid SCCmec-mecC element were capable of excision from the genome and forming a circular intermediate. Transcriptional analysis showed that mecC and mecA, but not mecA1, were both expressed in liquid culture supplemented with oxacillin.

CONCLUSIONS

Overall, this study further highlights that a range of staphylococcal species harbour the mecC gene and furthers the view that coagulase-negative staphylococci associated with animals may act as reservoirs of antibiotic resistance genes for more pathogenic staphylococcal species.

Properties of small rRNA methyltransferase RsmD: mutational and kinetic study

Ribosomal RNA modification is accomplished by a variety of enzymes acting on all stages of ribosome assembly. Among rRNA methyltransferases of Escherichia coli, RsmD deserves special attention. Despite its minimalistic domain architecture, it is able to recognize a single target nucleotide G966 of the 16S rRNA. RsmD acts late in the assembly process and is able to modify a completely assembled 30S subunit. Here, we show that it possesses superior binding properties toward the unmodified 30S subunit but is unable to bind a 30S subunit modified at G966. RsmD is unusual in its ability to withstand multiple amino acid substitutions of the active site. Such efficiency of RsmD may be useful to complete the modification of a 30S subunit ahead of the 30S subunit's involvement in translation.

Control of substrate specificity by a single active site residue of the KsgA methyltransferase

The KsgA methyltransferase is universally conserved and plays a key role in regulating ribosome biogenesis. KsgA has a complex reaction mechanism, transferring a total of four methyl groups onto two separate adenosine residues, A1518 and A1519, in the small subunit rRNA. This means that the active site pocket must accept both adenosine and N(6)-methyladenosine as substrates to catalyze formation of the final product N(6),N(6)-dimethyladenosine. KsgA is related to DNA adenosine methyltransferases, which transfer only a single methyl group to their target adenosine residue. We demonstrate that part of the discrimination between mono- and dimethyltransferase activity lies in a single residue in the active site, L114; this residue is part of a conserved motif, known as motif IV, which is common to a large group of S-adenosyl-L-methionine-dependent methyltransferases. Mutation of the leucine to a proline mimics the sequence found in DNA methyltransferases. The L114P mutant of KsgA shows diminished overall activity, and its ability to methylate the N(6)-methyladenosine intermediate to produce N(6),N(6)-dimethyladenosine is impaired; this is in contrast to a second active site mutation, N113A, which diminishes activity to a level comparable to L114P without affecting the methylation of N(6)-methyladenosine. We discuss the implications of this work for understanding the mechanism of KsgA's multiple catalytic steps.

Minimal substrate features for Erm methyltransferases defined by using a combinatorial oligonucleotide library

Erm methyltransferases are prevalent in pathogenic bacteria and confer resistance to macrolide, lincosamide, and streptogramin B antibiotics by specifically methylating the 23S ribosomal RNA at nucleotide A2058. We have identified motifs within the rRNA substrate that are required for methylation by Erm. Substrate molecules were constructed in a combinatorial manner from two separate sets (top and bottom strands) of short RNA sequences. Modifications, including LNA monomers with locked sugar residues, were incorporated into the substrates to stabilize their structures. In functional substrates, the A2058 methylation target (on the 13- to 19-nucleotide top strand) was displayed in an unpaired sequence immediately following a conserved irregular helix, and these are the specific structural features recognized by Erm. Erm methylation was enhanced by stabilizing the top-strand conformation with an LNA residue at G2056. The bottom strand (nine to 19 nucleotides in length) was required for methylation and was still functional after extensive modification, including substitution with a DNA sequence. Although it remains possible that Erm makes some unspecific contact with the bottom strand, the main role played by the bottom strand appears to be in maintaining the conformation of the top strand. The addition of multiple LNA residues to the top strand impeded methylation; this indicates that the RNA substrate requires a certain amount of flexibility for accommodation into the active site of Erm. The combinatorial approach for identifying small but functional RNA substrates is a step towards making RNA-Erm complexes suitable for cocrystal determination, and for designing molecules that might block the substrate-recognition site of the enzyme.

Assessment of clarithromycin susceptibility in strains belonging to the Mycobacterium abscessus group by erm(41) and rrl sequencing

Clarithromycin was the drug of choice for Mycobacterium abscessus infections until inducible resistance due to erm(41) was described. Because M. abscessus was split into M. abscessus sensu stricto, Mycobacterium massiliense, and Mycobacterium bolletii, we looked for erm(41) in the three species and determined their clarithromycin susceptibility levels. Ninety strains were included: 87 clinical strains from cystic fibrosis patients (61%) and others (39%), representing 43 M. abscessus, 30 M. massiliense, and 14 M. bolletii strains identified on a molecular basis, and 3 reference strains. Clarithromycin and azithromycin MICs were determined by broth microdilution and Etest with a 14-day incubation period. Mutations in rrl (23S rRNA gene) known to confer acquired clarithromycin resistance were also sought. erm(41) was detected in all strains but with two deletions in all M. massiliense strains. These strains were indeed susceptible to clarithromycin (MIC(90) of 1 μg/ml) except for four strains with rrl mutations. M. abscessus strains harbored an intact erm(41) but had a T/C polymorphism at the 28th nucleotide: T28 strains (Trp10 codon) demonstrated inducible clarithromycin resistance (MIC(90) of >16 μg/ml), while C28 strains (Arg10) were susceptible (MIC(90) of 2 μg/ml) except for two strains with rrl mutations. M. bolletii strains had erm(41) sequences similar to the sequence of the T28 M. abscessus group, associated with inducible clarithromycin resistance (MIC(90) of >16 μg/ml). erm(41) sequences appeared species specific within the M. abscessus group and were fully concordant with clarithromycin susceptibility when erm(41) sequencing was associated with detection of rrl mutations. Clarithromycin-resistant strains, including the six rrl mutants, were more often isolated in cystic fibrosis patients, but this was not significantly associated with a previous treatment.

Structural basis for the methylation of A1408 in 16S rRNA by a panaminoglycoside resistance methyltransferase NpmA from a clinical isolate and analysis of the NpmA interactions with the 30S ribosomal subunit

NpmA, a methyltransferase that confers resistance to aminoglycosides was identified in an Escherichia coli clinical isolate. It belongs to the kanamycin–apramycin methyltransferase (Kam) family and specifically methylates the 16S rRNA at the N1 position of A1408. We determined the structures of apo-NpmA and its complexes with S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) at 2.4, 2.7 and 1.68 Å, respectively. We generated a number of NpmA variants with alanine substitutions and studied their ability to bind the cofactor, to methylate A1408 in the 30S subunit, and to confer resistance to kanamycin in vivo. Residues D30, W107 and W197 were found to be essential. We have also analyzed the interactions between NpmA and the 30S subunit by footprinting experiments and computational docking. Helices 24, 42 and 44 were found to be the main NpmA-binding site. Both experimental and theoretical analyses suggest that NpmA flips out the target nucleotide A1408 to carry out the methylation. NpmA is plasmid-encoded and can be transferred between pathogenic bacteria; therefore it poses a threat to the successful use of aminoglycosides in clinical practice. The results presented here will assist in the development of specific NpmA inhibitors that could restore the potential of aminoglycoside antibiotics.

Binding of adenosine-based ligands to the MjDim1 rRNA methyltransferase: implications for reaction mechanism and drug design

The KsgA/Dim1 family of proteins is intimately involved in ribosome biogenesis in all organisms. These enzymes share the common function of dimethylating two adenosine residues near the 3'-OH end of the small subunit rRNA; orthologs in the three kingdoms, along with eukaryotic organelles, have evolved additional functions in rRNA processing, ribosome assembly, and, surprisingly, transcription in mitochondria. The methyltransferase reaction is intriguingly elaborate. The enzymes can bind to naked small subunit rRNA but cannot methylate their target bases until a subset of ribosomal proteins have bound and the nascent subunit has reached a certain level of maturity. Once this threshold is reached, the enzyme must stabilize two adenosines into the active site at separate times and two methyl groups must be transferred to each adenosine, with concomitant exchanges of the product S-adenosyl-l-homocysteine and the methyl donor substrate S-adenosyl-l-methionine. A detailed molecular understanding of this mechanism is currently lacking. Structural analysis of the interactions between the enzyme and substrate will aid in this understanding. Here we present the structure of KsgA from Methanocaldococcus jannaschii in complex with several ligands, including the first structure of S-adenosyl-l-methionine bound to a KsgA/Dim1 enzyme in a catalytically productive way. We also discuss the inability thus far to determine a structure of a target adenosine bound in its active site.

Structural basis for binding of RNA and cofactor by a KsgA methyltransferase

Among methyltransferases, KsgA and the reaction it catalyzes are conserved throughout evolution. However, the specifics of substrate recognition by the enzyme remain unknown. Here we report structures of Aquifex aeolicus KsgA, in its ligand-free form, in complex with RNA, and in complex with both RNA and S-adenosylhomocysteine (SAH, reaction product of cofactor S-adenosylmethionine), revealing critical structural information on KsgA-RNA and KsgA-SAH interactions. Moreover, the structures show how conformational changes that occur upon RNA binding create the cofactor-binding site. There are nine conserved functional motifs (motifs I-VIII and X) in KsgA. Prior to RNA binding, motifs I and VIII are flexible, each exhibiting two distinct conformations. Upon RNA binding, the two motifs become stabilized in one of these conformations, which is compatible with the binding of SAH. Motif X, which is also stabilized upon RNA binding, is directly involved in the binding of SAH.

Structural rearrangements in the active site of the Thermus thermophilus 16S rRNA methyltransferase KsgA in a binary complex with 5'-methylthioadenosine

Posttranscriptional modification of ribosomal RNA (rRNA) occurs in all kingdoms of life. The S-adenosyl-L-methionine-dependent methyltransferase KsgA introduces the most highly conserved rRNA modification, the dimethylation of A1518 and A1519 of 16S rRNA. Loss of this dimethylation confers resistance to the antibiotic kasugamycin. Here, we report biochemical studies and high-resolution crystal structures of KsgA from Thermus thermophilus. Methylation of 30S ribosomal subunits by T. thermophilus KsgA is more efficient at low concentrations of magnesium ions, suggesting that partially unfolded RNA is the preferred substrate. The overall structure is similar to that of other methyltransferases but contains an additional alpha-helix in a novel N-terminal extension. Comparison of the apoenzyme with complex structures with 5'-methylthioadenosine or adenosine bound in the cofactor-binding site reveals novel features when compared with related enzymes. Several mobile loop regions that restrict access to the cofactor-binding site are observed. In addition, the orientation of residues in the substrate-binding site indicates that conformational changes are required for binding two adjacent residues of the substrate rRNA.

Sequence-structure-function analysis of the bifunctional enzyme MnmC that catalyses the last two steps in the biosynthesis of hypermodified nucleoside mnm5s2U in tRNA

MnmC catalyses the last two steps in the biosynthesis of 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U) in tRNA. Previously, we reported that this bifunctional enzyme is encoded by the yfcK open reading frame in the Escherichia coli K12 genome. However, the mechanism of its activity, in particular the potential structural and functional dependence of the domains responsible for catalyzing the two modification reactions, remains unknown. With the aid of the protein fold-recognition method, we constructed a structural model of MnmC in complex with the ligands and target nucleosides and studied the role of individual amino acids and entire domains by site-directed and deletion mutagenesis, respectively. We found out that the N-terminal domain contains residues responsible for binding of the S-adenosylmethionine cofactor and catalyzing the methylation of nm(5)s(2)U to form mnm(5)s(2)U, while the C-terminal domain contains residues responsible for binding of the FAD cofactor. Further, point mutants with compromised activity of either domain can complement each other to restore a fully functional enzyme. Thus, in the conserved fusion protein MnmC, the individual domains retain independence as enzymes. Interestingly, the N-terminal domain is capable of independent folding, while the isolated C-terminal domain is incapable of folding on its own, a situation similar to the one reported recently for the rRNA modification enzyme RsmC.

Subcellular localization and RNA interference of an RNA methyltransferase gene from silkworm, Bombyx mori

RNA methylation, which is a form of posttranscriptional modification, is catalyzed by S-adenosyl-L-methionone-dependent RNA methyltransterases (RNA MTases). We have identified a novel silkworm gene, BmRNAMTase, containing a 369-bp open reading frame that encodes a putative protein containing 122 amino acid residues and having a molecular weight of 13.88 kd. We expressed a recombinant His-tagged BmRNAMTase in E. coli BL21 (DE3), purified the fusion protein by metal-chelation affinity chromatography, and injected a New Zealand rabbit with the purified protein to generate anti-BmRNAMTase polyclonal antibodies. Immunohistochemistry revealed that BmRNAMTase is abundant in the cytoplasm of Bm5 cells. In addition, using RNA interference to reduce the intracellular activity and content of BmRNAMTase, we determined that this cytoplasmic RNA methyltransferase may be involved in preventing cell death in the silkworm.

Bud23 methylates G1575 of 18S rRNA and is required for efficient nuclear export of pre-40S subunits

BUD23 was identified from a bioinformatics analysis of Saccharomyces cerevisiae genes involved in ribosome biogenesis. Deletion of BUD23 leads to severely impaired growth, reduced levels of the small (40S) ribosomal subunit, and a block in processing 20S rRNA to 18S rRNA, a late step in 40S maturation. Bud23 belongs to the S-adenosylmethionine-dependent Rossmann-fold methyltransferase superfamily and is related to small-molecule methyltransferases. Nevertheless, we considered that Bud23 methylates rRNA. Methylation of G1575 is the only mapped modification for which the methylase has not been assigned. Here, we show that this modification is lost in bud23 mutants. The nuclear accumulation of the small-subunit reporters Rps2-green fluorescent protein (GFP) and Rps3-GFP, as well as the rRNA processing intermediate, the 5' internal transcribed spacer 1, indicate that bud23 mutants are defective for small-subunit export. Mutations in Bud23 that inactivated its methyltransferase activity complemented a bud23Delta mutant. In addition, mutant ribosomes in which G1575 was changed to adenosine supported growth comparable to that of cells with wild-type ribosomes. Thus, Bud23 protein, but not its methyltransferase activity, is important for biogenesis and export of the 40S subunit in yeast.

Crystal structure of Thermus thermophilus tRNA m1A58 methyltransferase and biophysical characterization of its interaction with tRNA

Methyltransferases from the m(1)A(58) tRNA methyltransferase (TrmI) family catalyze the S-adenosyl-l-methionine-dependent N(1)-methylation of tRNA adenosine 58. The crystal structure of Thermus thermophilus TrmI, in complex with S-adenosyl-l-homocysteine, was determined at 1.7 A resolution. This structure is closely related to that of Mycobacterium tuberculosis TrmI, and their comparison enabled us to enlighten two grooves in the TrmI structure that are large enough and electrostatically compatible to accommodate one tRNA per face of TrmI tetramer. We have then conducted a biophysical study based on electrospray ionization mass spectrometry, site-directed mutagenesis, and molecular docking. First, we confirmed the tetrameric oligomerization state of TrmI, and we showed that this protein remains tetrameric upon tRNA binding, with formation of complexes involving one to two molecules of tRNA per TrmI tetramer. Second, three key residues for the methylation reaction were identified: the universally conserved D170 and two conserved aromatic residues Y78 and Y194. We then used molecular docking to position a N(9)-methyladenine in the active site of TrmI. The N(9)-methyladenine snugly fits into the catalytic cleft, where the side chain of D170 acts as a bidentate ligand binding the amino moiety of S-adenosyl-l-methionine and the exocyclic amino group of the adenosine. Y194 interacts with the N(9)-methyladenine ring, whereas Y78 can stabilize the sugar ring. From our results, we propose that the conserved residues that form the catalytic cavity (D170, Y78, and Y194) are essential for fashioning an optimized shape of the catalytic pocket.

Modeling and experimental analyses reveal a two-domain structure and amino acids important for the activity of aminoglycoside resistance methyltransferase Sgm

Methyltransferases that carry out posttranscriptional N7-methylation of G1405 in 16S rRNA confer bacterial resistance to aminoglycoside antibiotics, including kanamycin and gentamicin. Genes encoding enzymes from this family (hereafter referred to as Arm, for aminoglycoside resistance methyltransferases) have been recently found to spread by horizontal gene transfer between various human pathogens. The knowledge of the Arm protein structure would lay the groundwork for the development of potential resistance inhibitors, which could be used to restore the potential of aminoglycosides to act against the resistant pathogens. We analyzed the sequence-function relationships of Sgm MTase, a member of the Arm family, by limited proteolysis and site-directed and random mutagenesis. We also modeled the structure of Sgm using bioinformatics techniques and used the model to provide a structural context for experimental results. We found that Sgm comprises two domains and we characterized a number of functionally compromised point mutants with substitutions of invariant or conserved residues. Our study provides a low-resolution (residue-level) model of sequence-structure-function relationships in the Arm family of enzymes and reveals the cofactor-binding and substrate-binding sites. These functional regions will be prime targets for further experimental and theoretical studies aimed at defining the reaction mechanism of m7 G1405 methylation, increasing the resolution of the model and developing Arm-specific inhibitors.

2-Aminopurine Flipped into the Active Site of the Adenine-Specific DNA Methyltransferase M.TaqI: Crystal Structures and Time-Resolved Fluorescence

We report the crystal structure of the DNA adenine-N6 methyltransferase, M.TaqI, complexed with DNA, showing the fluorescent adenine analog, 2-aminopurine, flipped out of the DNA helix and occupying virtually the same position in the active site as the natural target adenine. Time-resolved fluorescence spectroscopy of the crystalline complex faithfully reports this state: base flipping is accompanied by the loss of the very short ( approximately 50 ps) lifetime component associated with fully base-stacked 2-aminopurine in DNA, and 2-aminopurine is subject to considerable quenching by pi-stacking interactions with Tyr108 in the catalytic motif IV (NPPY). This proves 2-aminopurine to be an excellent probe for studying base flipping by M.TaqI and suggests similar quenching in the active sites of DNA and RNA adenine-N6 as well as DNA cytosine-N4 methyltransferases sharing the conserved motif IV. In solution, the same distinctive fluorescence response confirms complete destacking from DNA and is also observed when the proposed key residue for base flipping by M.TaqI, the target base partner thymine, is substituted by an abasic site analog. The corresponding cocrystal structure shows 2-aminopurine in the active site of M.TaqI, demonstrating that the partner thymine is not essential for base flipping. However, in this structure, a shift of the 3' neighbor of the target base into the vacancy left after base flipping is observed, apparently replicating a stabilizing role of the missing partner thymine. Time-resolved fluorescence and acrylamide quenching measurements of M.TaqI complexes in solution provide evidence for an alternative binding site for the extra-helical target base within M.TaqI and suggest that the partner thymine assists in delivering the target base into the active site.

Catalysis by the second class of tRNA(m1G37) methyl transferase requires a conserved proline

The enzyme tRNA(m1G37) methyl transferase catalyzes the transfer of a methyl group from S-adenosyl methionine (AdoMet) to the N1 position of G37, which is 3' to the anticodon sequence and whose modification is important for maintaining the reading frame fidelity. While the enzyme in bacteria is highly conserved and is encoded by the trmD gene, recent studies show that the counterpart of this enzyme in archaea and eukarya, encoded by the trm5 gene, is unrelated to trmD both in sequence and in structure. To further test this prediction, we seek to identify residues in the second class of tRNA(m1G37) methyl transferase that are required for catalysis. Such residues should provide mechanistic insights into the distinct structural origins of the two classes. Using the Trm5 enzyme of the archaeon Methanocaldococcus jannaschii (previously MJ0883) as an example, we have created mutants to test many conserved residues for their catalytic potential and substrate-binding capabilities with respect to both AdoMet and tRNA. We identified that the proline at position 267 (P267) is a critical residue for catalysis, because substitution of this residue severely decreases the kcat of the methylation reaction in steady-state kinetic analysis, and the k(chem) in single turnover kinetic analysis. However, substitution of P267 has milder effect on the Km and little effect on the Kd of either substrate. Because P267 has no functional side chain that can directly participate in the chemistry of methyl transfer, we suggest that its role in catalysis is to stabilize conformations of enzyme and substrates for proper alignment of reactive groups at the enzyme active site. Sequence analysis shows that P267 is embedded in a peptide motif that is conserved among the Trm5 family, but absent from the TrmD family, supporting the notion that the two families are descendants of unrelated protein structures.

Methyltransferase Erm(37) slips on rRNA to confer atypical resistance in Mycobacterium tuberculosis

Members of the Mycobacterium tuberculosis complex possess a resistance determinant, erm(37) (also termed ermMT), which is a truncated homologue of the erm genes found in a diverse range of drug-producing and pathogenic bacteria. All erm genes examined thus far encode N(6)-monomethyltransferases or N(6),N(6)-dimethyltransferases that show absolute specificity for nucleotide A2058 in 23 S rRNA. Monomethylation at A2058 confers resistance to a subset of the macrolide, lincosamide, and streptogramin B (MLS(B)) group of antibiotics and no resistance to the latest macrolide derivatives, the ketolides. Dimethylation at A2058 confers high resistance to all MLS(B) and ketolide drugs. The erm(37) phenotype fits into neither category. We show here by tandem mass spectrometry that Erm(37) initially adds a single methyl group to its primary target at A2058 but then proceeds to attach additional methyl groups to the neighboring nucleotides A2057 and A2059. Other methyltransferases, Erm(E) and Erm(O), maintain their specificity for A2058 on mycobacterial rRNA. Erm(E) and Erm(O) have a full-length C-terminal domain, which appears to be important for stabilizing the methyltransferases at their rRNA target, and this domain is truncated in Erm(37). The lax interaction of the M. tuberculosis Erm(37) with its rRNA produces a unique methylation pattern and confers resistance to the ketolide telithromycin.

Structural and functional characterization of CFE88: evidence that a conserved and essential bacterial protein is a methyltransferase

CFE88 is a conserved essential gene product from Streptococcus pneumoniae. This 227-residue protein has minimal sequence similarity to proteins of known 3D structure. Sequence alignment models and computational protein threading studies suggest that CFE88 is a methyltransferase. Characterization of the conformation and function of CFE88 has been performed by using several techniques. Backbone atom and limited side-chain atom NMR resonance assignments have been obtained. The data indicate that CFE88 has two domains: an N-terminal domain with 163 residues and a C-terminal domain with 64 residues. The C-terminal domain is primarily helical, while the N-terminal domain has a mixed helical/extended (Rossmann) fold. By aligning the experimentally observed elements of secondary structure, an initial unrefined model of CFE88 has been constructed based on the X-ray structure of ErmC' methyltransferase (Protein Data Bank entry 1QAN). NMR and biophysical studies demonstrate binding of S-adenosyl-L-homocysteine (SAH) to CFE88; these interactions have been localized by NMR to the predicted active site in the N-terminal domain. Mutants that target this predicted active site (H26W, E46R, and E46W) have been constructed and characterized. Overall, our results both indicate that CFE88 is a methyltransferase and further suggest that the methyltransferase activity is essential for bacterial survival.

Drosophila Mitochondrial Transcription Factor B1 Modulates Mitochondrial Translation but Not Transcription or DNA Copy Number in Schneider Cells

We report the cloning and molecular analysis of Drosophila mitochondrial transcription factor (d-mtTF) B1. An RNA interference (RNAi) construct was designed that reduces expression of d-mtTFB1 to 5% of its normal level in Schneider cells. In striking contrast with our previous study on d-mtTFB2, we found that RNAi knock-down of d-mtTFB1 does not change the abundance of specific mitochondrial RNA transcripts, nor does it affect the copy number of mitochondrial DNA. In a corollary manner, overexpression of d-mtTFB1 did not increase either the abundance of mitochondrial RNA transcripts or mitochondrial DNA copy number. Our data suggest that, unlike d-mtTFB2, d-mtTFB1 does not have a critical role in either transcription or regulation of the copy number of mitochondrial DNA. Instead, because we found that RNAi knockdown of d-mtTFB1 reduces mitochondrial protein synthesis, we propose that it serves its primary role in modulating translation. Our work represents the first study to document the role of mtTFB1 in vivo and establishes clearly functional differences between mtTFB1 and mtTFB2.

Structural and Thermodynamic Evidence for a Stabilizing Role of Nop5p in S-Adenosyl-L-methionine Binding to Fibrillarin

In Archaea, fibrillarin and Nop5p form the core complex of box C/D small ribonucleoprotein particles, which are responsible for site-specific 2'-hydroxyl methylation of ribosomal and transfer RNAs. Fibrillarin has a conserved methyltransferase fold and employs S-adenosyl-l-methionine (AdoMet) as the cofactor in methyl transfer reactions. Comparison between recently determined crystal structures of free fibrillarin and fibrillarin-Nop5p-AdoMet tertiary complex revealed large conformational differences at the cofactor-binding site in fibrillarin. To identify the structural elements responsible for these large conformational differences, we refined a crystal structure of Archaeoglobus fulgidus fibrillarin-Nop5p binary complex at 3.5 A. This structure exhibited a pre-formed backbone geometry at the cofactor binding site similar to that when the cofactor is bound, suggesting that binding of Nop5p alone to fibrillarin is sufficient to stabilize the AdoMet-binding pocket. Calorimetry studies of cofactor binding to fibrillarin alone and to fibrillarin-Nop5p binary complex provided further support for this role of Nop5p. Mutagenesis and thermodynamic data showed that a cation-pi bridge formed between Tyr-89 of fibrillarin and Arg-169 of Nop5p, although dispensable for in vitro methylation activity, could partially account for the enhanced binding of cofactor to fibrillarin by Nop5p. Finally, assessment of cofactor-binding thermodynamics and catalytic activities of enzyme mutants identified three additional fibrillarin residues (Thr-70, Glu-88, and Asp-133) to be important for cofactor binding and for catalysis.

Crystal structure of KsgA, a universally conserved rRNA adenine dimethyltransferase in Escherichia coli

The bacterial enzyme KsgA catalyzes the transfer of a total of four methyl groups from S-adenosyl-l-methionine (S-AdoMet) to two adjacent adenosine bases in 16S rRNA. This enzyme and the resulting modified adenosine bases appear to be conserved in all species of eubacteria, eukaryotes, and archaebacteria, and in eukaryotic organelles. Bacterial resistance to the aminoglycoside antibiotic kasugamycin involves inactivation of KsgA and resulting loss of the dimethylations, with modest consequences to the overall fitness of the organism. In contrast, the yeast ortholog, Dim1, is essential. In yeast, and presumably in other eukaryotes, the enzyme performs a vital role in pre-rRNA processing in addition to its methylating activity. Another ortholog has been discovered recently, h-mtTFB in human mitochondria, which has a second function; this enzyme is a nuclear-encoded mitochondrial transcription factor. The KsgA enzymes are homologous to another family of RNA methyltransferases, the Erm enzymes, which methylate a single adenosine base in 23S rRNA and confer resistance to the MLS-B group of antibiotics. Despite their sequence similarity, the two enzyme families have strikingly different levels of regulation that remain to be elucidated. We have crystallized KsgA from Escherichia coli and solved its structure to a resolution of 2.1A. The structure bears a strong similarity to the crystal structure of ErmC' from Bacillus stearothermophilus and a lesser similarity to sc-mtTFB, the Saccharomyces cerevisiae version of h-mtTFB. Comparison of the three crystal structures and further study of the KsgA protein will provide insight into this interesting group of enzymes.

Insights into catalysis by a knotted TrmD tRNA methyltransferase

The crystal structure of Escherichia coli tRNA (guanosine-1) methyltransferase (TrmD) complexed with S-adenosyl homocysteine (AdoHcy) has been determined at 2.5A resolution. TrmD, which methylates G37 of tRNAs containing the sequence G36pG37, is a homo-dimer. Each monomer consists of a C-terminal domain connected by a flexible linker to an N-terminal AdoMet-binding domain. The two bound AdoHcy moieties are buried at the bottom of deep clefts. The dimer structure appears integral to the formation of the catalytic center of the enzyme and this arrangement strongly suggests that the anticodon loop of tRNA fits into one of these clefts for methyl transfer to occur. In addition, adjacent hydrophobic sites in the cleft delineate a defined pocket, which may accommodate the GpG sequence during catalysis. The dimer contains two deep trefoil peptide knots and a peptide loop extending from each knot embraces the AdoHcy adenine ring. Mutational analyses demonstrate that the knot is important for AdoMet binding and catalytic activity, and that the C-terminal domain is not only required for tRNA binding but plays a functional role in catalytic activity.