Binding of erythromycin to Escherichia coli ribosomes

Partial release of AcPhe-Phe-tRNA from ribosomes during poly(U)-dependent poly(Phe) synthesis and the effects of chloramphenicol

Poly(U)-programmed 70S ribosomes can be shown to be 80% to 100% active in binding the peptidyl-tRNA analogue AcPhe-tRNA to their A or P sites, respectively. Despite this fact, only a fraction of such ribosomes primed with AcPhe-tRNA participate in poly(U)-directed poly(Phe) synthesis (up to 65%) at 14 mM Mg2+ and 160 mM NH4+. Here it is demonstrated that the apparently 'inactive' ribosomes (greater than or equal to 35%) are able to participate in peptide-bond formation, but lose their nascent peptidyl-tRNA at the stage of Ac(Phe)n-tRNA, with n greater than or equal to 2. The relative loss of early peptidyl-tRNAs is largely independent of the degree of initial saturation with AcPhe-tRNA and is observed in a poly(A) system as well. This observation resolves a current controversy concerning the active fraction of ribosomes. The loss of Ac(Phe)n-tRNA is reduced but still significant if more physiological conditions for Ac(Phe)n synthesis are applied (3 mM Mg2+, 150 mM NH4+, 2 mM spermidine, 0.05 mM spermine). Chloramphenicol (0.1 mM) blocks the puromycin reaction with AcPhe-tRNA as expected but, surprisingly, does not affect the puromycin reaction with Ac(Phe)2-tRNA nor peptide bond formation between AcPhe-tRNA and Phe-tRNA. The drug facilitates the release of Ac(Phe)2-4-tRNA from ribosomes at 14 mM Mg2+ while it hardly affects the overall synthesis of poly(Phe) or poly(Lys).

Erythromycin induces migrating motor complex in human gastrointestinal tract

Fifteen healthy subjects, fasted at least 8 hr, were studied by means of an infused manometric method. Twenty minutes after termination of the natural phase III activity in the duodenum, erythromycin or normal saline was administered intravenously for 15 min. When normal saline (N = 5) was infused, the next migrating motor complex (MMC) was initiated 151.2 +/- 42.1 min after the infusion. On the other hand, when erythromycin was infused at a rate of 1.0 mg/kg/hr (N = 5) or 3.0 mg/kg/hr (N = 5), MMC-like contractions were initiated at shorter intervals, ie, 47.8 +/- 40.9 min (P less than 0.005) or 23.0 +/- 13.0 min (P less than 0.001) respectively. The duration, frequency, amplitude, and migrating velocity of the naturally occurring MMC (N-MMC) were not significantly different from those of the erythromycin-induced contractions except for the duration of the phase III contractions in the stomach; the duration (5.3 +/- 2.2 min) of the erythromycin-induced contractions being significantly (P less than 0.05) longer than that (3.2 +/- 0.9 min) of the naturally occurring MMC. The immunoreactive motilin (IRM) concentration did not increase significantly after the infusion of erythromycin, when compared to that after infusion of normal saline. It is concluded that erythromycin at a dose of 1-3 mg/kg/hr for 15 min during the interdigestive state, similar to motilin, has a significant influence upon the initiation of MMC in the human gastrointestinal tract, but further investigations are required to confirm whether endogenous motilin is involved or not.

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.

Erythromycin, carbomycin, and spiramycin inhibit protein synthesis by stimulating the dissociation of peptidyl-tRNA from ribosomes

In mutant Escherichia coli with temperature-sensitive peptidyl-tRNA hydrolase (aminoacyl-tRNA hydrolase; EC 3.1.1.29), peptidyl-tRNA accumulates at the nonpermissive temperature (40 degrees C), and the cells die. These consequences of high temperature were enhanced if the cells were first treated with erythromycin, carbomycin, or spiramycin at doses sufficient to inhibit protein synthesis in wild-type cells but not sufficient to kill either mutant or wild-type cells at the permissive temperature (30 degrees C). Since peptidyl-tRNA hydrolase in he mutant cells is inactivated rapidly and irreversibly at 40 degrees C, the enhanced accumulation of peptidyl-tRNA and killing were the result of enhanced dissociation, stimulated by the antibiotics, of peptidyl-tRNA from ribosomes. The implications of these findings for inhibition of cell growth and protein synthesis are discussed. Certain alternative interpretations are shown to be inconsistent with the relevant data. Previous conflicting observations on the effects of macrolide antibiotics are explained in terms of our observations. We conclude that erythromycin, carbomycin, and spiramycin (and probably all macrolides) have as a primary mechanism of action the stimulation of dissociation of peptidyl-tRNA from ribosomes, probably during translocation.

Posttranscriptional modification of mRNA conformation: mechanism that regulates erythromycin-induced resistance

The nucleotide sequence of a gene in plasmid pE194 responsible for erythromycin-induced resistance, including regulation of the resistance phenotype, is reported. A DNA fragment from plasmid pE194, obtained by digestion with Taq I restriction endonuclease, was cloned in Bacillus subtilis by using pC194 as the plasmid cloning vector. Erythromycin-resistant, inducible transformant clones containing the Taq I fragment A were obtained in which the expression of resistance was similar to that found in the original pE194 background; an interpretative model of the regulation of the erythromycin-resistance determinant is proposed based on the sequence of the Taq I A fragment. The cloned Taq I A fragment consists of 1442 base pairs and has open reading frames capable of coding for a peptide and a protein containing 19 and 243 amino acids, respectively, referred to as the "leader peptide" and "29,000 protein." Between the putative transcriptional start site and the ribosome binding site for 29,000-protein synthesis, the promoter region contains four complementary inverted repeat sequences named "1, 2, 3, and 4," respectively, in which 1 is complementary to 2, 2 is complementary to 3, and 3 is complementary to 4. Sequence 1 encodes the COOH-terminal half of the leader peptide, whereas the ribosome binding site for synthesis of 29,000 protein is sequestered in a loop formed by the association of 3 and 4. The 29,000-protein promoter region does not appear to contain any transcription stop signal. We propose a model for regulation of erythromycin resistance according to which ribosomes engaged in leader peptide synthesis are partially inhibited by optimal inducing (i.e., subinhibitory) concentrations of erythromycin that, in turn, cause an accumulation of these partially inhibited ("stalled") ribosomes in sequence 1. During induction, the translationally inactive states of association of the inverted repeats, postulated to be 1 plus 2 and 3 plus 4, respectively, are perturbed by a high level of stalled ribosome occupancy in sequence 1, and in the resultant redistribution, 2 associates with 3, freeing 4 and thereby freeing the ribosome binding site sequestered by the association of 3 and 4. Sequence alterations at the 5' end of the 29,000-protein coding region associated with mutation to constitutive expression have been localized to the inverted complementary repeats, and determination of base changes in eight mutants are all capable of reducing the stability of the postulated stems in a manner consistent with predictions made by the model.

Ribosomal function and its inhibition by antibiotics in prokaryotes

Most of the known antibiotics act at the level of protein biosynthesis probably due to the extraordinary complexity of the translation machinery which can be interfered with at many points. At first a survey is given of our present knowledge covering the structure and function of the prokaryotic ribosome. The most important antibiotics acting at the translational level are integrated into this network of data. The binding sites and the inhibition mechanisms of the drugs, together with the ribosomal components altered in resistant mutants are described. Finally, the points of interference with the translational machinery are indicated in an extended scheme of ribosomal functions.

Erythromycin-resistant mutant of Escherichia coli with altered ribosomal protein component

Erythromycin combines with 50S ribosomal subunit of an erythromycin-sensitive Escherichia coli (strain Q13), while ribosomes from an erythromycin-resistant mutant from this strain have little affinity for the antibiotic. A protein component of the 50S subunit of the mutant strain is distinct from that of the parent Q13 strain.

Accumulation in gram-postive and gram-negative bacteria as a mechanism of resistance to erythromycin

Erythromycin was recovered in high yield after incubation with gram-negative bacteria. The cell-free protein-synthesizing preparation from gram-negative bacteria is equally as susceptible to the antibiotic as is that from gram-positive bacteria. Thus, neither destruction of erythromycin nor the absence of the step susceptible to the antibiotic plays an important role in the resistance mechanism of gram-negative bacteria. A 100-fold difference in accumulation of erythromycin between gram-positive and gram-negative bacteria was observed. This alone explains the resistance of gram-negative bacteria to erythromycin. Furthermore, data showed that the inhibition of growth is closely related to the accumulation of erythromycin. The concentration of intracellular erythromycin in gram-positive bacteria was found to be 44- to 90-fold greater than that of the extracellular medium. However, the antibiotic did not accumulate on the cell walls, nor was the accumulation energy-dependent. It is proposed that it takes place by the binding of erythromycin to the bacterial ribosomes, forming a very stable complex. The dissociation constants of erythromycin-Staphylococcus aureus complex and erythromycin-Bacillus subtilis complex were determined to be 1.1 x 10(-7) and 3.4 x 11(-7)m, respectively.