Genetic studies of he ribosoml proteis in Escherichaoli. V. Mapp-ing of erythromycin resistance mutations which lea to alteration of a 50s ribosomal protein component

Mechanisms of multidrug transporters

Drug resistance, mediated by various mechanisms, plays a crucial role in the failure of the drug-based treatment of various infectious diseases. As a result, these infectious diseases re-emerge rapidly and cause many victims every year. Another serious threat is imposed by the development of multidrug resistance (MDR) in eukaryotic (tumor) cells, where many different drugs fail to perform their therapeutic function. One of the causes of the occurrence of MDR in these cells is the action of transmembrane transport proteins that catalyze the active extrusion of a large number of structurally and functionally unrelated compounds out of the cell. The mode of action of these MDR transporters and their apparent lack of substrate specificity is poorly understood and has been subject to many speculations. In this review we will summarize our current knowledge about the occurrence, mechanism and molecular basis of (multi-)drug resistance especially as found in bacteria.

Erythromycin inhibits the assembly of the large ribosomal subunit in growing Escherichia coli cells

Erythromycin and other macrolide antibiotics have been examined for their effects on ribosome assembly in growing Escherichia coli cells. Formation of the 50S ribosomal subunit was specifically inhibited by erythromycin and azithromycin. Other related compounds tested, including oleandomycin, clarithromycin, spiramycin, and virginiamycin M1, did not influence assembly. Erythromycin did not promote the breakdown of ribosomes formed in the absence of the drug. Two erythromycin-resistant mutants with alterations in ribosomal proteins L4 and L22 were also examined for an effect on assembly. Subunit assembly was affected in the mutant containing the L22 alteration only at erythromycin concentrations fourfold greater than those needed to stop assembly in wild-type cells. Ribosomal subunit assembly was only marginally affected at the highest drug concentration tested in the cells that contained the altered L4 protein. These novel results indicate that erythromycin has two effects on translation, preventing elongation of the polypeptide chain and also inhibiting the formation of the large ribosomal subunit.

Identification of mutations in 23S rRNA gene of clarithromycin-resistant Mycobacterium intracellulare

Clarithromycin is a potent macrolide that has been used for treating infections with nontuberculous mycobacteria. Pairs of susceptible and resistant Mycobacterium intracellulare strains were obtained from patients with chronic pulmonary M. intracellulare infections undergoing monotherapy with clarithromycin. Nucleotide sequence comparisons of the peptidyltransferase region in 23S rRNAs from parental and resistant strains revealed that in three of six resistant strains, for which the MIC was > 32 micrograms/ml, a single base was mutated (Escherichia coli equivalent, A-2058-->G, C, or U). As the modification of adenine 2058 by dimethylation is a frequent cause of macrolide resistance in a variety of different bacteria, we suggest that mutation of A-2058 confers acquired resistance to clarithromycin in M. intracellulare.

Translational attenuation: the regulation of bacterial resistance to the macrolide-lincosamide-streptogramin B antibiotics

The regulation of ermC is described in detail as an example of regulation on the level of translation. ermC specifies a ribosomal RNA methylase which confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics. Synthesis of the ermC gene product is induced by erythromycin, a macrolide antibiotic. Stimulation of methylase synthesis is mediated by binding of erythromycin to an unmethylated ribosome. The translational attenuation model, supported by sequencing data and by mutational analysis, proposes that binding of erythromycin causes stalling of a ribosome during translation of a "leader peptide", resulting in isomerization of the ermC transcript from an inactive to an active conformer. The ermC system is analogous to the transcriptional attenuation systems described for certain biosynthetic operons. ermC is unique in that interaction with a small molecule inducer mediates regulation on the translational level. However, it is but one example of nontranscriptional -level control of protein synthesis. Other systems are discussed in which control is also exerted through alterations of RNA conformation and an attempt is made to understand ermC in this more general context. Finally, other positive examples of translational attenuation are presented.

Mutation to erythromycin dependence in Escherichia coli K-12

A nitrosoguanidine-induced mutant of Escherichia coli K-12 strain JC12 was absolutely dependent on erythromycin or related macrolide antibiotics for growth. The only other drugs which permitted growth (lincomycin and chloramphenicol) are, like the macrolides, inhibitors of the 50S ribosome. The order of relative effectiveness of these drugs was macrolides > lincomycin > chloramphenicol. Rates of growth with all drugs were concentration dependent. Erythromycin starvation was followed by normal rates of increase in cell mass and macromolecular synthesis for approximately one mass-doubling time, after which macromolecular synthesis abruptly ceased and cell lysis and death occurred. The dependent mutant gave rise spontaneously to revertants to independence with very high frequency (10(-4)). The gene (mac) for macrolide dependence is located near minute 25 on the E. coli chromosome; it does not result in increased resistance to these drugs. A separate gene for erythromycin resistance (eryA) is located in the cluster of ribosomal structural genes near spc, close to minute 63. Dependence on macrolides was most clearly evident in strains carrying mutations at both eryA and mac.

Chromosomal localization of the structural genes of the polypeptide chain elongation factors

A survey of the polypeptide chain elongation factors in potentially sexually compatible genera was carried out. Factors from Escherichia coli and Proteus mirabilis were found to be clearly distinguishable by immunochemical and electrophoretic techniques. Mapping of the structural genes of these factors was undertaken by a study of the gene products in genetically defined E. coli-P. mirabilis hybrid diploid strains. It was found that the EF G factor mapped within 5 min of the streptomycin resistance locus, but the EF Ts factor did not map in this region.