23S rRNA domain V, a fragment that can be specifically methylated in vitro by the ErmSF (TlrA) methyltransferase

Characteristics of the ErmK Protein of Bacillus halodurans C-125

Bacillus halodurans C-125 is an alkaliphilic microorganism that grows best at pH 10 to 10.5. B. halodurans C-125 harbors the erm (erythromycin resistance methylase) gene as well as the mphB (macrolide phosphotransferase) and putative mef (macrolide efflux) genes, which confer resistance to macrolide, lincosamide, and streptogramin B (MLSB) antibiotics. The Erm protein expressed in B. halodurans C-125 could be classified as ErmK because it shares 66.2% and 61.2% amino acid sequence identity with the closest ErmD and Erm(34), respectively. ErmK can be regarded as a dimethylase, as evidenced by reverse transcriptase analysis and the antibiotic resistance profile exhibited by E. coli expressing ermK. Although ErmK showed one-third or less in vitro methylating activity compared to ErmC', E. coli cells expressing ErmK exhibited comparable resistance to erythromycin and tylosin, and a similar dimethylation proportion of 23S rRNA due to the higher expression rate in a T7 promoter-mediated expression system. The less efficient methylation activity of ErmK might reflect an adaption to mitigate the fitness cost caused by dimethylation through the Erm protein presumably because B. halodurans C-125 has less probability to encounter the antibiotics in its favorable growth conditions and grows retardedly in neutral environments. IMPORTANCE Erm proteins confer MLSB antibiotic resistance (minimal inhibitory concentration [MIC] value up to 4,096 μg/mL) on microorganisms ranging from antibiotic producers to pathogens, imposing one of the most pressing threats to clinics. Therefore, Erm proteins have long been speculated to be plausible targets for developing inhibitor(s). In our laboratory, it has been noticed that there are variations in enzymatic activity among the Erm proteins, Erm in antibiotic producers being better than that in pathogens. In this study, it has been observed that Erm protein in B. halodurans C-125 extremophile is a novel member of Erm protein and acts more laggardly, compared to that in pathogen. While this sluggishness of Erm protein in extremophile might be evolved to reduce the fitness cost incurred by Erm activity adapting to its environments, this feature could be exploited to develop the more potent and/or efficacious drug to combat formidably problematic antibiotic-resistant pathogens.

Identification of Allosteric Hotspots regulating the ribosomal RNA-binding by Antibiotic Resistance-Conferring Erm Methyltransferases

Antibiotic resistance via epigenetic methylation of ribosomal RNA is one of the most prevalent strategies adopted by multidrug resistant pathogens. The erythromycin-resistance methyltransferase (Erm) methylates rRNA at the conserved A2058 position and imparts resistance to macrolides such as erythromycin. However, the precise mechanism adopted by Erm methyltransferases for locating the target base within a complicated rRNA scaffold remains unclear. Here, we show that a conserved RNA architecture, including specific bulge sites, present more than 15 Å from the reaction center, is key to methylation at the pathogenic site. Using a set of RNA sequences site-specifically labeled by fluorescent nucleotide surrogates, we show that base flipping is a prerequisite for effective methylation and that distal bases assist in the recognition and flipping at the reaction center. The Erm–RNA complex model revealed that intrinsically flipped-out bases in the RNA serve as a putative anchor point for the Erm. Molecular dynamic simulation studies demonstrated the RNA undergoes a substantial change in conformation to facilitate an effective protein–rRNA handshake. This study highlights the importance of unique architectural features exploited by RNA to impart fidelity to RNA methyltransferases via enabling allosteric crosstalk. Moreover, the distal trigger sites identified here serve as attractive hotspots for the development of combination drug therapy aimed at reversing resistance.

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.

Plausible Minimal Substrate for Erm Protein

Erm proteins methylate a specific adenine residue (A2058, Escherichia coli coordinates) conferring macrolide-lincosamide-streptogramin B (MLSB) antibiotic resistance on a variety of microorganisms, ranging from antibiotic producers to pathogens. To identify the minimal motif required to be recognized and methylated by the Erm protein, various RNA substrates from 23S rRNA were constructed, and the substrate activity of these constructs was studied using three Erm proteins, namely, ErmB from Firmicutes and ErmE and ErmS from Actinobacteria The shortest motif of 15 nucleotides (nt) could be recognized and methylated by ErmS, consisting of A2051 to the methylatable adenine (A2058) and its base-pairing counterpart strand, presumably assuming a quite similar structure to that in 23S rRNA, an unpaired target adenine immediately followed by an irregular double-stranded RNA region. This observation confirms the ultimate end of each side in helix 73 for methylation, determined by the approaches described above, and could reveal the mechanism behind the binding, recognition, induced fit, methylation, and conformational change for product release in the minimal context of substrate, presumably with the help of structural determination of the protein-RNA complex. In the course of determining the minimal portion of substrate from domain V, protein-specific features could be observed among the Erm proteins in terms of the methylation of RNA substrate and cooperativity and/or allostery between the region in helix 73 furthest away from the target adenine and the large portion of domain V above the methylatable adenine.

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.

YgdE is the 2'-O-ribose methyltransferase RlmM specific for nucleotide C2498 in bacterial 23S rRNA

The rRNAs of Escherichia coli contain four 2'-O-methylated nucleotides. Similar to other bacterial species and in contrast with Archaea and Eukaryota, the E. coli rRNA modifications are catalysed by specific methyltransferases that find their nucleotide targets without being guided by small complementary RNAs. We show here that the ygdE gene encodes the methyltransferase that catalyses 2'-O-methylation at nucleotide C2498 in the peptidyl transferase loop of E. coli 23S rRNA. Analyses of rRNAs using MALDI mass spectrometry showed that inactivation of the ygdE gene leads to loss of methylation at nucleotide C2498. The loss of ygdE function causes a slight reduction in bacterial fitness. Methylation at C2498 was restored by complementing the knock-out strain with a recombinant copy of ygdE. The recombinant YgdE methyltransferase modifies C2498 in naked 23S rRNA, but not in assembled 50S subunits or ribosomes. Nucleotide C2498 is situated within a highly conserved and heavily modified rRNA sequence, and YgdE's activity is influenced by other modification enzymes that target this region. Phylogenetically, YgdE is placed in the cluster of orthologous groups COG2933 together with S-adenosylmethionine-dependent, Rossmann-fold methyltransferases such as the archaeal and eukaryotic RNA-guided fibrillarins. The ygdE gene has been redesignated rlmM for rRNA large subunit methyltransferase M.

The activity of rRNA resistance methyltransferases assessed by MALDI mass spectrometry

Resistance to antibiotics that target the bacterial ribosome is often conferred by methylation at specific nucleotides in the rRNA. The nucleotides that become methylated are invariably key sites of antibiotic interaction. The addition of methyl groups to each of these nucleotides is catalyzed by a specific methyltransferase enzyme. The Erm methyltransferases are a clinically prevalent group of enzymes that confer resistance to the therapeutically important macrolide, lincosamide, and streptogramin B (MLS B) antibiotics. The target for Erm methyltransferases is at nucleotide A2058 in 23S rRNA, and methylation occurs before the rRNA has been assembled into 50S ribosomal particles. Erm methyltransferases occur in a phylogenetically wide range of bacteria and differ in whether they add one or two methyl groups to the A2058 target. The dimethylated rRNA confers a more extensive MLS B resistance phenotype. We describe here a method using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to determine the location and number of methyl groups added at any site in the rRNA. The method is particularly suited to studying in vitro methylation of RNA transcripts by resistance methyltransferases such as Erm.

Macrolide resistance

The macrolides have evolved through four chemical generations since erythromycin became available for clinical use in 1952. The first generation, the 14-membered ring macrolide erythromycin, induced resistance and was replaced by the second generation 16-membered ring macrolides which did not. The inability to induce came at the price of mutation, in the pathogenic target strain, to constitutive expression of resistance. A third generation of macrolides improved the acid-stability, and therefore the pharmacokinetics of erythromycin, extending the clinical use of macrolides to Helicobacter pylori and Mycobacterium tuberculosis. Improved pharmacokinetics resulted in the selection of intrinsically resistant mutant strains with rRNA structural alterations. Expression of resistance in these strains was unexpected, explainable by low rRNA gene copy number which made resistance dominant. A fourth generation of macrolides, the 14-membered ring ketolides are the most recent development. Members of this generation are reported to be effective against inducibly resistant strains, and ketolide resistant strains have not yet been reported. In this review we discuss details of the ways in which bacteria have become resistant to the first three generations of macrolides, both with respect to their biochemistry, and the genetic mechanisms by which their expression is regulated.

Purification and biochemical characterization of the ErmSF macrolide-lincosamide-streptogramin B resistance factor protein expressed as a hexahistidine-tagged protein in Escherichia coli

The erm proteins confer resistance to the MLS (macrolide-lincosamide-streptogramin B) antibiotics in various microorganisms, including pathogens, through dimethylation of a single adenine residue (A2085: Bacillus subtilis coordinate) of the 23S rRNA to reduce the affinity of antibiotics, thereby enabling the cells to escape from the antibiotics' action, and this mechanism is predominantly adopted by microorganisms resistant to MLS antibiotics. ErmSF methyltransferase is one of the four gene products synthesized by Streptomyces fradiae NRRL 2338 to be resistant to its autogenous antibiotic, tylosin. In order to have a convenient source for the purification of milligram amounts, we expressed ErmSF in Escherichia coli using a T7 promoter-driven expression vector system, pET 23b, and the protein was expressed with a carboxy-terminal addition of six histidine residues in order to facilitate purification. Expression at 22 degrees C reduced the formation of insoluble aggregate, inclusion body, and resulted in accumulation of soluble hexahistidine-ErmSF up to 30% of total cell protein after 18 h. Metal-chelation chromatography yielded 126 mg of hexahistidine-ErmSF per liter of culture with a purity slightly greater than 95%. To examine the function of ErmSF in vivo and in vitro, its activity in E. coli (antibiotic susceptibility assay) andin vitro methyltransferase activity using in vitro-produced B. subtilis domain V, 434-, 257-, and 243-nt RNAs were investigated. The ErmSF in E. coli conferred resistance to erythromycin, whereas E. coli harboring an empty vector, pET23b, was susceptible. The purified recombinant protein successfully methylated domain V of 23S rRNA, which is known to contain all of the substrate elements recognized and to be methylated by erm proteins. However, the truncated substrates were methylated with decreased efficiencies. Almost all of domain V was monomethylated with less than 0.2 pM S-[methyl-(3)H]adenosylmethionine concentration. The roles of three structurally divided regions of domain V in recognition and methylation by ErmSF are proposed through kinetic studies using RNA substrates, in which each region is deleted, under the monomethylation condition.

Activity of the ketolide telithromycin is refractory to Erm monomethylation of bacterial rRNA

Methylation of specific nucleotides in rRNA is one of the means by which bacteria achieve resistance to macrolides-lincosamides-streptogramin B (MLS(B)) and ketolide antibiotics. The degree of resistance is determined by how effectively the rRNA is methylated. We have implemented a bacterial system in which the rRNA methylations are defined, and in this study we investigate what effect Erm mono- and dimethylation of the rRNA has on the activity of representative MLS(B) and ketolide antibiotics. In the test system, >80% of the rRNA molecules are monomethylated by ErmN (TlrD) or dimethylated by ErmE. ErmE dimethylation confers high resistance to all the MLS(B) and ketolide drugs. ErmN monomethylation predictably confers high resistance to the lincosamides clindamycin and lincomycin, intermediate resistance to the macrolides clarithromycin and erythromycin, and low resistance to the streptogramin B pristinamycin IA. In contrast to the macrolides, monomethylation only mildly affects the antimicrobial activities of the ketolides HMR 3647 (telithromycin) and HMR 3004, and these drugs remain 16 to 250 times as potent as clarithromycin and erythromycin. These differences in the macrolide and ketolide activities could explain the recent reports of variation in the MICs of telithromycin for streptococcal strains that have constitutive erm MLS(B) resistance and are highly resistant to erythromycin.

Modulation of erm methyltransferase activity by peptides derived from phage display

Combinatorial peptide display on phage M13 protein pIII was used to discover peptide sequences that selectively bind to ErmC' methyltransferase from Bacillus subtilis. One peptide, Ac-LSGVIAT-NH(2), inhibited methylation in vitro with a 50% inhibitory concentration of 20 microM. Interestingly, the set of six peptides which inhibited ErmC' stimulated ErmSF, a homologous methyltransferase from Streptomyces fradiae. Thus, Ac-LSGVIAT-NH(2) may not act directly at the catalytic center of ErmC', but may modulate its activity by binding at a structurally unrelated, but functionally linked, site.

Negative in vitro selection identifies the rRNA recognition motif for ErmE methyltransferase

Erm methyltransferases modify bacterial 23S ribosomal RNA at adenosine 2058 (A2058, Escherichia coli numbering) conferring resistance to macrolide, lincosamide, and streptogramin B (MLS) antibiotics. The motif that is recognized by Erm methyltransferases is contained within helix 73 of 23S rRNA and the adjacent single-stranded region around A2058. An RNA transcript of 72 nt that displays this motif functions as an efficient substrate for the ErmE methyltransferase. Pools of degenerate RNAs were formed by doping 34-nt positions that extend over and beyond the putative Erm recognition motif within the 72-mer RNA. The RNAs were passed through a series of rounds of methylation with ErmE. After each round, RNAs were selected that had partially or completely lost their ability to be methylated. After several rounds of methylation/selection, 187 subclones were analyzed. Forty-three of the subclones contained substitutions at single sites, and these are confined to 12 nucleotide positions. These nucleotides, corresponding to A2051-A2060, C2611, and A2614 in 23S rRNA, presumably comprise the RNA recognition motif for ErmE methyltransferase. The structure formed by these nucleotides is highly conserved throughout bacterial rRNAs, and is proposed to constitute the motif that is recognized by all the Erm methyltransferases.

The 2.2 A structure of the rRNA methyltransferase ErmC' and its complexes with cofactor and cofactor analogs: implications for the reaction mechanism

The rRNA methyltransferase ErmC' transfers methyl groups from S -adenosyl-l-methionine to atom N6 of an adenine base within the peptidyltransferase loop of 23 S rRNA, thus conferring antibiotic resistance against a number of macrolide antibiotics. The crystal structures of ErmC' and of its complexes with the cofactor S -adenosyl-l-methionine, the reaction product S-adenosyl-l-homocysteine and the methyltransferase inhibitor Sinefungin, respectively, show that the enzyme undergoes small conformational changes upon ligand binding. Overall, the ligand molecules bind to the protein in a similar mode as observed for other methyltransferases. Small differences between the binding of the amino acid parts of the different ligands are correlated with differences in their chemical structure. A model for the transition-state based on the atomic details of the active site is consistent with a one-step methyl-transfer mechanism and might serve as a first step towards the design of potent Erm inhibitors.

ErmE methyltransferase recognizes features of the primary and secondary structure in a motif within domain V of 23 S rRNA

The Erm methyltransferases confer resistance to macrolide, lincosamide and streptogramin B (MLS) antibiotics by methylation of a single adenosine base within bacterial 23 S ribosomal RNA. The ErmE methyltransferase, from the macrolide-producing bacterium Saccharopolyspora erythraea, recognizes a motif within domain V of the rRNA that specifically targets adenosine 2058 (A2058) for methylation. Here, we define the structure of the RNA motif by a combination of molecular genetics and biochemical probing. The core of the motif has the primary sequence 2056-GGAHA-2060, where H is any nucleotide except guanosine, and ErmE methylates at the adenosine in bold. For efficient recognition by ErmE, this sequence must be displayed within a particular secondary structure. An irregular stem (helix 73) is required immediately 5' to A2058, with an unpaired nucleotide, preferably a cytidine residue, at position 2055. Nucleotides 2611 to 2616 are collectively required to form part of the 3'-side of helix 73, but there is little or no restriction on the identities of individual nucleotides here. There are minor preferences in the identities of nucleotides 2051 to 2055 that are adjacent to the motif core, although their main role is in maintaining the irregular secondary structure. The essential elements of the ErmE motif are conserved in bacterial 23 S rRNAs, and thus presumably also form the recognition motif for other Erm methyltransferases.

Dissecting the VanRS signal transduction pathway with specific inhibitors

The VanRS two-component signal transduction pathway from Enterococcus faecium was reconstituted in vitro from partially purified components and shown to be inhibited by the halophenyl isothiazolone LY-266,400, inhibitor A, a compound shown previously to reduce expression of the AlgR1-AlgR2 two-component signal transduction pathway in Pseudomonas aeruginosa (S. Roychoudhury, N. A. Zielinski, A. J. Ninfa, N. E. Allen, L. N. Jungheim, T. I. Nicas, and A. M. Chakrabarty, Proc. Natl. Acad. Sci. USA 90:965-969, 1993). Inhibitor A attenuates phosphoryl transfer from VanS approximately P to VanR by its action on the ability of VanR to accept. We observed an apparent stimulatory effect of inhibitor A on VanS autophosphorylation which is attributable to the accumulation of VanS approximately P as an intermediate unable to transfer Pi to the inhibited VanR. Thus, inhibitor A acts on the second of two sequential steps which lead to transcriptional activation of the VanHAXYZ gene cluster and the resultant expression of vancomycin resistance.

ErmE methyltransferase recognition elements in RNA substrates

Dimethylation by Erm methyltransferases at the N-6 position of adenine 2058 (A2058, Escherichia coli numbering) in domain V of bacterial 23 S rRNA confers resistance to the macrolide-lincosamide-streptogramin B (MLS) group of antibiotics. The ErmE methyltransferase from Saccharopolyspora erythraea methylates a 625 nucleotide transcript of domain V as efficiently as it methylates intact 23 S rRNA. By progressively truncating domain V, the motif required for specific recognition by the enzyme has been localized to a helix and single-stranded region adjacent to A2058. The smallest RNA transcript that shows methyl-accepting activity is a 27-nucleotide stem-loop, corresponding to the 23 S rRNA sequences 2048 to 2063 and 2610 to 2620 (helix 73), with A2058 situated within the hairpin loop. Methylation of A2058 in the truncated RNAs is optimal in the absence of magnesium, and the efficiency of methylation is halved by the presence of 2 to 3 mM magnesium. Magnesium serves to stabilize a conformation in the truncated RNA that prevents efficient methylation. This contrasts to the intact domain V RNA, where 2 mM magnesium ions support a conformation at A2058 that is most readily recognized by ErmE. Methylation of domain V RNA is generally far less susceptible to ionic conditions than the truncated RNAs. The effects of monovalent cations on the methylation of truncated transcripts suggest that RNA structures outside helix 73 support the ErmE interaction. However, interaction with these structures is not essential for specific ErmE recognition of A2058.

Essential role of endogenously synthesized tylosin for induction of ermSF in Streptomyces fradiae

We compared ermSF induction in wild-type Streptomyces fradiae NRRL B-2702 and that in GS-14, a tylA mutant which cannot synthesize tylosin. Our findings suggest that (i) endogenously synthesized tylosin plays an obligatory role in ermSF induction and (ii) tylosin, or a biosynthetic intermediate beyond tylactone, has an "autocrine" function that induces ErmSF synthesis, thereby enabling S. fradiae to resist higher levels of tylosin.

Ribosome regulation by the nascent peptide

Studies of bacterial and eukaryotic systems have identified two-gene operons in which the translation product of the upstream gene influences translation of the downstream gene. The upstream gene, referred to as a leader (gene) in bacterial systems or an upstream open reading frame (uORF) in eukaryotes, encodes a peptide that interferes with a function(s) of its translating ribosome. The peptides are therefore cis-acting negative regulators of translation. The inhibitory peptides typically consist of fewer than 25 residues and function prior to emergence from the ribosome. A biological role for this class of translation inhibitor is demonstrated in translation attenuation, a form or regulation that controls the inducible translation of the chloramphenicol resistance genes cat and cmlA in bacteria. Induction of cat or cmlA requires ribosome stalling at a particular codon in the leader region of the mRNA. Stalling destabilizes an adjacent, downstream mRNA secondary structure that normally sequesters the ribosome-binding site for the cat or cmlA coding regions. Genetic studies indicate that the nascent, leader-encoded peptide is the selector of the site of ribosome stalling in leader mRNA by cis interference with translation. Synthetic leader peptides inhibit ribosomal peptidyltransferase in vitro, leading to the prediction that this activity is the basis for stall site selection. Recent studies have shown that the leader peptides are rRNA-binding peptides with targets at the peptidyl transferase center of 23S rRNA. uORFs associated with several eukaryotic genes inhibit downstream translation. When inhibition depends on the specific codon sequence of the uORF, it has been proposed that the uORF-encoded nascent peptide prevents ribosome release from the mRNA at the uORF stop codon. This sets up a blockade to ribosome scanning which minimizes downstream translation. Segments within large proteins also appear to regulate ribosome activity in cis, although in most of the known examples the active amino acid sequences function after their emergence from the ribosome, cis control of translation by the nascent peptide is gene specific; nearly all such regulatory peptides exert no obvious trans effects in cells. The in vitro biochemical activities of the cat/cmla leader peptides on ribosomes and rRNA suggest a mechanism through which the nascent peptide can modify ribosome behavior. Other cis-acting regulatory peptides may involve more complex ribosomal interactions.

Recognition determinants for proteins and antibiotics within 23S rRNA

Ribosomal RNAs fold into phylogenetically conserved secondary and tertiary structures that determine their function in protein synthesis. We have investigated Escherichia coli 23S rRNA to identify structural elements that interact with antibiotic and protein ligands. Using a combination of molecular genetic and biochemical probing techniques, we have concentrated on regions of the rRNA that are connected with specific functions. These are located in different domains within the 23S rRNA and include the ribosomal GTPase-associated center in domain II, which contains the binding sites for r-proteins L10.(L12)4 and L11 and is inhibited by interaction with the antibiotic thiostrepton. The peptidyltransferase center within domain V is inhibited by macrolide, lincosamide, and streptogramin B antibiotics, which interact with the rRNA around nucleotide A2058. Drug resistance is conferred by mutations here and by modification of A2058 by ErmE methyltransferase. ErmE recognizes a conserved motif displayed in the primary and secondary structure of the peptidyl transferase loop. Within domain VI of rRNA, the alpha-sarcin stem-loop is associated with elongation factor binding and is the target site for ribotoxins including the N-glycosidase ribosome-inactivating proteins ricin and pokeweed antiviral protein (PAP). The orientations of the 23S rRNA domains are constrained by tetiary interactions, including a pseudoknot in domain II and long-range base pairings in the center of the molecule that bring domains II and V closer together. The phenotypic effects of mutations in these regions have been investigated by expressing 23S rRNA from plasmids. Allele-specific priming sites have been introduced close to these structures in the rRNA to enable us to study the molecular events there.

Substrate requirements for ErmC' methyltransferase activity

ErmC' is a methyltransferase that confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics by catalyzing the methylation of 23S rRNA at a specific adenine residue (A-2085 in Bacillus subtilis; A-2058 in Escherichia coli). The gene for ErmC' was cloned and expressed to a high level in E. coli, and the protein was purified to virtual homogeneity. Studies of substrate requirements of ErmC' have shown that a 262-nucleotide RNA fragment within domain V of B. subtilis 23S rRNA can be utilized efficiently as a substrate for methylation at A-2085. Kinetic studies of the monomethylation reaction showed that the apparent Km of this 262-nucleotide RNA oligonucleotide was 26-fold greater than the value determined for full-size and domain V 23S rRNA. In addition, the Vmax for this fragment also rose sevenfold. A model of RNA-ErmC' interaction involving multiple binding sites is proposed from the kinetic data presented.

The conformation of 23S rRNA nucleotide A2058 determines its recognition by the ErmE methyltransferase

The ErmE methyltransferase confers resistance to MLS antibiotics by specifically dimethylating adenine 2058 (A2058, Escherichia coli numbering) in bacterial 23S rRNA. To define nucleotides in the rRNA that are part of the motif recognized by ErmE, we investigated both in vivo and in vitro the effects of mutations around position A2058 on methylation. Mutagenizing A2058 (to G or U) completely abolishes methylation of 23S rRNA by ErmE. No methylation occurred at other sites in the rRNA, demonstrating the fidelity of ErmE for A2058. Breaking the neighboring G2057-C2611 Watson-Crick base pair by introducing either an A2057 or a U2611 mutation, greatly reduces the rate of methylation at A2058. Methylation remains impaired after these mutations have been combined to create a new A2057-U2611 Watson-Crick base interaction. The conformation of this region in 23S rRNA was probed with chemical reagents and it was shown that the A2057 and U2611 mutations alone and in combination alter the reactivity of A2058 and adjacent bases. However, mutagenizing position G-->A2032 in an adjacent loop, which has been implicated to interact with A2058, alters neither the ErmE methylation at A2058 nor the accessibility of this region to the chemical reagents. The data indicate that a less-exposed conformation at A2058 leads to reduction in methylation by ErmE. Nucleotide G2057 and its interaction with C2611 maintain the conformation at A2058, and are thus important in forming the structural motif that is recognized by the ErmE methyltransferase.