No abstract

Macrolides represent a clinically important class of antibiotics that block protein synthesis by interacting with the large ribosomal subunit. The macrolide binding site is composed primarily of rRNA. However, the mode of interaction of macrolides with rRNA and the exact location of the drug binding site have yet to be described. A new class of macrolide antibiotics, known as ketolides, show improved activity against organisms that have developed resistance to previously used macrolides. The biochemical reasons for increased potency of ketolides remain unknown. Here we describe the first mutation that confers resistance to ketolide antibiotics while leaving cells sensitive to other types of macrolides. A transition of U to C at position 2609 of 23S rRNA rendered E. coli cells resistant to two different types of ketolides, telithromycin and ABT-773, but increased slightly the sensitivity to erythromycin, azithromycin, and a cladinose-containing derivative of telithromycin. Ribosomes isolated from the mutant cells had reduced affinity for ketolides, while their affinity for erythromycin was not diminished. Possible direct interaction of ketolides with position 2609 in 23S rRNA was further confirmed by RNA footprinting. The newly isolated ketolide-resistance mutation, as well as 23S rRNA positions shown previously to be involved in interaction with macrolide antibiotics, have been modeled in the crystallographic structure of the large ribosomal subunit. The location of the macrolide binding site in the nascent peptide exit tunnel at some distance from the peptidyl transferase center agrees with the proposed model of macrolide inhibitory action and explains the dominant nature of macrolide resistance mutations. Spatial separation of the rRNA residues involved in universal contacts with macrolides from those believed to participate in structure-specific interactions with ketolides provides the structural basis for the improved activity of the broader spectrum group of macrolide antibiotics.Macrolide antibiotics inhibit protein synthesis in a wide range of pathogenic bacteria. The drugs bind to a single site in the large ribosomal subunit located near the entrance to the nascent peptide tunnel and are thought to sterically hinder the growth of the polypeptide chain and cause dissociation of peptidyl-tRNA from the ribosome (1, 10, 18, 33). Macrolide antibiotics were also shown to affect ribosome assembly (9).The macrolide binding site is composed primarily of RNA. Two segments of 23S rRNA, the central loop of domain V and the loop of hairpin 35 from domain II, are believed to be the major components of the drug binding site on the ribosome. Mutations of A2058 (Escherichia coli numeration) and several neighboring positions in the central loop of domain V confer resistance to macrolides (27), as reviewed in references 34 and 35. Methylation of A2058 by Erm-type methyl-transferases drastically reduces the affinity of macrolides for the ribosome, suggesting the direct interaction between this RNA residue and the drug (11,1...

The Erm family of methyltransferases is responsible for the development of resistance to the macrolide-lincosamide-streptogramin type B (MLS) antibiotics. These enzymes methylate an adenine of 23S ribosomal RNA that prevents the MLS antibiotics from binding to the ribosome and exhibiting their antibacterial activity. Here we describe the three-dimensional structure of an Erm family member, ErmAM, as determined by NMR spectroscopy. The catalytic domain of ErmAM is structurally similar to that found in other methyltransferases and consists of a seven-stranded beta-sheet flanked by alpha-helices and a small two-stranded beta-sheet. In contrast to the catalytic domain, the substrate binding domain is different from other methyltransferases and adopts a novel fold that consists of four alpha-helices.

Background Transmission of HIV drug resistance (TDR) gives rise to reduced efficacy of initial antiretroviral treatment, and has become a public health concern. Methods A nationwide survey on TDR was conducted in antiretroviral therapy naïve HIV-1 infected individuals from September 2004 to October 2005 in China. Drug resistance genotyping was performed on subjects’ plasma samples. Drug resistance mutations were determined and scored by Stanford HIV Drug Resistance algorithm. Results Sequences were obtained from 676 individuals, of which 61.2% were former plasma and blood donors, 17.3% were infected sexually, and 17.2% were intravenous drug users. Subtype B’ HIV-1 strains were found in 73.5%, CRF01_AE in 13.9%, CRF07_BC in 6.2%, CRF08_BC in 2.7%, subtype C in 1.04%, subtype B in 0.9%, CRF02_AG in 0.4% and B’/C intersubtype recombinant strains in 1.3% of the subjects. Twenty-six (3.8%) were found to harbor drug resistance strains. The rates of resistance to PIs, NRTIs and NNRTIs were 0.4%, 1.6% and 2.1%, respectively. Though there was no significant difference in TDR rates between 2004 and 2005 (2.9% vs. 4.4%), an increased trend was observed in the rate of high-level drug resistance (0.8% in 2004 vs. 3.0% in 2005, P = 0.0634). Conclusions The rate of TDR was relatively low in China, as compared with those in developed countries. Surveys among recently HIV-infected subjects should be preformed continually to ensure the success of the scale-up antiretroviral treatment.