Insights into protein biosynthesis and ribosome function through inhibitors

The use of erythromycin as a gastrointestinal prokinetic agent in adult critical care: benefits versus risks

Erythromycin A, the first macrolide, was introduced in the 1950s and after years of clinical experience it still remains a commonly relied upon antibiotic. In the past, pharmacodynamic characteristics of macrolides beyond antimicrobial action such as anti-inflammatory and immune-modulating properties have been of scientific and clinical interest. The function of erythromycin as a prokinetic agent has also been investigated for a range of gastrointestinal motility disorders and more recently within the context of critically ill patients. Prokinetic agents are drugs that increase contractile force and accelerate intraluminal transit. Whilst the anti-inflammatory action may be a desirable side effect to its antibiotic action, using erythromycin A merely for its prokinetic effect alone raises the concern about promoting emergence of macrolide resistance. The objectives of this review article are: (i) to briefly summarize the modes and epidemiology of macrolide resistance, particularly in respect to that found in the Streptococcus species (a potential reservoir for the dissemination of macrolide resistance on the critical care unit); (ii) to discuss in this context the evidence for conditions promoting bacterial resistance against macrolides; and (iii) to assess the potential clinical benefit of using erythromycin A as a prokinetic versus the risks of promoting emergence of macrolide resistance in the clinical setting. We conclude, that in view of the growing weight of evidence demonstrating the potential epidemiological impact of the increased use of macrolides upon the spread of resistance, versus a lack of sufficient and convincing evidence that erythromycin A is a superior prokinetic agent to potential alternatives in the critically ill patient population, at this stage we do not advocate the use of erythromycin A as a prokinetic agent in critically ill patients unless they have failed all other treatment for impaired gastrointestinal dysmotility and are intolerant of metoclopramide. Further large and methodologically robust studies are needed to ascertain the effectiveness of erythromycin A and other alternative agents in the critically ill.

EmtA, a rRNA methyltransferase conferring high-level evernimicin resistance

Enterococcus faecium strain 9631355 was isolated from animal sources on the basis of its resistance to the growth promotant avilamycin. The strain also exhibited high-level resistance to evernimicin, a drug undergoing evaluation as a therapeutic agent in humans. Ribosomes from strain 9631355 exhibited a dramatic reduction in evernimicin binding, shown by both cell-free translation assays and direct-binding assays. The resistance determinant was cloned from strain 9631355; sequence alignments suggested it was a methyltransferase and therefore it was designated emtA for evernimicin methyltransferase. Evernimicin resistance was transmissible and emtA was localized to a plasmid-borne insertion element. Purified EmtA methylated 50S subunits from an evernimicin-sensitive strain 30-fold more efficiently than those from a resistant strain. Reverse transcription identified a pause site that was unique to the 23S rRNA extracted from resistant ribosomes. The pause corresponded to methylation of residue G2470 (Escherichia coli numbering). RNA footprinting revealed that G2470 is located within the evernimicin-binding site on the ribosome, thus providing an explanation for the reduced binding of the drug to methylated ribosomes.

Antisense RNA: effect of ribosome binding sites, target location, size, and concentration on the translation of specific mRNA molecules

A series of plasmids were constructed to generate RNA complementary to the beta-galactosidase messenger RNA under control of the phage lambda PL promoter. These plasmids generate anti-lacZ mRNA bearing or lacking a synthetic ribosome binding site adjacent to the lambda PL promoter and/or the lacZ ribosome binding site in reverse orientation. Fragments of lacZ DNA from the 5' and/or the 3' region were used in these constructions. When these anti-mRNA molecules were produced in Escherichia coli 294, maximal inhibition of beta-galactosidase synthesis occurred when a functional ribosome binding site was present near the 5' end of the anti-mRNA and the anti-mRNA synthesized was complementary to the 5' region of the mRNA corresponding to the lacZ ribosome binding site and/or the 5'-coding sequence. Anti-mRNAs producing maximal inhibition of beta-galactosidase synthesis exhibited an anti-lacZ mRNA:normal lacZ mRNA ratio of 100:1 or higher. Those showing lower levels of inhibition exhibited much lower anti-lacZ mRNA:normal lacZ mRNA ratios. A functional ribosome binding site at the 5'-end was found to decrease the decay rate of the anti-lacZ mRNAs. In addition, the incorporation of a transcription terminator just downstream of the antisense segment provided for more efficient inhibition of lacZ mRNA translation due to synthesis of smaller and more abundant anti-lacZ mRNAs. The optimal constructions produced undetectable levels of beta-galactosidase synthesis.

Aminoacylaminonucleoside inhibitors of protein synthesis. A new approach for evaluating their potency

In a model system derived from Escherichia coli, Ac[3H]Phe-puromycin is produced in a pseudo-first-order reaction between the preformed Ac[3H]Phe-tRNA-poly(U)-ribosome complex (complex C) and excess puromycin [Kalpaxis et al. Eur. J. Biochem. 154, 267, 1986]. Amicetin and gougerotin inhibit this reaction to various degrees depending on whether or not complex C is allowed to interact with the inhibitor (I) prior to the addition of puromycin (S). The kinetic analysis shows a phase where competitive inhibition can be observed provided that S and I are added simultaneously. After preincubating C with I, the inhibition becomes of the mixed non-competitive type. The Ki (the dissociation constant of the CI complex), calculated from the competitive plot, is 20.0 microM for amicetin and 15.0 microM for gougerotin. This inhibition constant (Ki) cannot distinguish amicetin from gougerotin. Its acceptance as a criterion of potency does not explain why after preincubation amicetin proves to be a stronger inhibitor than gougerotin. The determination of the apparent catalytic rate constants of peptidyltransferase at various inhibitor concentrations and the appropriate replotting of these rate constants distinguish amicetin from gougerotin. A new approach for evaluating the potency of these inhibitors is proposed. The familiar Ki is supplemented with an apparent kinetic constant obtained from a replot in which the intercepts of the double-reciprocal plots (1/kobs versus 1/[S]) are plotted versus the inhibitor concentration.

Inhibitory action of virginiamycin components on cell-free systems for polypeptide formation from Bacillus subtilis

Although virginiamycin components VM and VS are known to exert in vivo a synergistic inhibition of bacterial growth and viability, in cell-free systems only VM has proven active. In the present work, the in vivo and in vitro activities of VM and VS on Bacillus subtilis have been compared. Peptide formation in homogenates of bacteria previously incubated with either VM or VS was found strongly repressed; the 2 components acted synergistically. Ribosomes were fully responsible for this effect, as shown by mixed reconstitution experiments. On the other hand, cytoplasm from control bacteria disrupted in 10 mM Mg2+ buffer was refractory to in vitro inhibition by virginiamycin, whereas ribosomes prepared in 1 mM Mg2+ were sensitive to VM. VS was inactive on poly(U)-directed poly(phenylalanine) formation, and displayed some activity on the poly(A)-poly(lysine) system. In a cell-free system from Bacillus subtilis infected with phage 2C, both VM and VS were active and blocked synergistically protein synthesis in vitro. When the host cells were incubated with VS and the corresponding homogenate was then treated with VM, a complete inhibition of protein synthesis was observed. The present work, thus, describes the techniques for investigating the in vivo and in vitro action of synergimycins on the same organism, and for reproducing in vitro the synergistic interaction of type A and B components previously observed only in vivo.

The lasting ribosome alteration produced by virginiamycin M disappears upon removal of certain ribosomal proteins

Transient incubation of bacterial ribosomes with virginiamycin M produces a lasting damage of 50 S ribosomal subunits, whereby the elongation of peptide chains is still blocked after removal of the antibiotic. To elucidate the mechanism of this inactivation, ribosomal proteins were stepwise removed from 50 S subunits previously incubated with virginiamycin M, and cores were submitted to three functional tests. Total removal of proteins L7, L8, L12 and L16, and partial removal of L6, L9, L10 and L11, resulted in a loss of the virginiamycin M-induced alteration. When the split protein fractions were added back to these cores, unaltered functional particles were obtained. The reconstituted subunits, on the other hand, proved fully sensitive to virginiamycin M in vitro as they underwent, upon transient contact with the antibiotic, an alteration comparable to that of native particles. It is concluded that the virginiamycin M-induced ribosome damage is due to the production of a stable conformational change of the 50 S subunit. These data parallel those of an accompanying paper (Cocito, C., Vanlinden, F. and Branlant, C. (1983) Biochim. Biophys. Acta 739, 158-163) showing the intactness of all rRNA species from ribosomes treated in vivo and in vitro with virginiamycin M.

Initial inhibition and recovery of protein synthesis in cycloheximide-treated hepatocytes

Previous studies conducted with intact rats had demonstrated that protein synthesis was reversibly inhibited by cycloheximide. Polysome aggregation occurred during inhibition with a return to normal during recovery. Suggesting that the block of translational activity involved termination and release of polypeptides. This study involving freshly isolated hepatocytes was undertaken to clarify the mechanism of the biphasic response to cycloheximide. Cycloheximide at 1 microM inhibited [3H]leucine incorporation into both cellular and secreted proteins by at least 86%, without having deleterious effects on membrane integrity as indicated by trypan blue uptake and lactate dehydrogenase (LDH) (EC 1.1.1.27) release. After removal of cycloheximide, incorporation of labeled amino acids into cellular protein and protein secreted into the medium returned to control levels. Kinetically, incorporation into secreted protein exhibited a lag of 30-45 min, indicating that a longer recovery period for restoration of proteosynthetic ability is required for membrane-bound polysomes. During the first 100 min of the recovery period, 30% of the cellular protein, which had been prelabeled during cycloheximide inhibition, was secreted into the medium; treated cells, however, secreted prelabeled protein at a lower initial rate. To elucidate the mechanism of action of cycloheximide, the content of the cytoplasmic ribonucleoprotein complexes (RPC), polysome size classes, and the distribution of radioactivity among the various ribosome classes were determined during inhibition and recovery. Larger size class polysomes (7+) were increased by cycloheximide treatment and remained increased during recovery. During inhibition, there was enhanced [3H]leucine labeling with increasing polysome size, implicating termination as the rate-limiting step, whereas during the recovery phase the labeled nascent polypeptides were removed from the ribonucleoprotein complex at a 3- to 4-fold greater rate than control, indicating an accelerated release of completed proteins.

Characterisation of the binding of virginiamycin S to Escherichia coli ribosomes

Virginiamycin S is an inhibitor of protein synthesis in vivo. In this paper we show by equilibrium dialysis that it binds specifically to the 50-S subunit of Escherichia coli ribosomes, with one binding site per subunit. This binding is not altered by the presence of chloramphenicol, tetracycline or puromycin but is competed for by erythromycin. Using the splitting-reconstitution method, it could be demonstrated that protein L16 is absolutely required for the binding of virginiamycin S to the 50-S subunit.

The concept of cellular evolution

A central evolutionary question is whether the eucaryotic cytoplasm represents a line of descent that is separate from the typical bacterial line. It is argued on the basis of differences between their respective translation mechanisms that the two lines do represent separate phylogenetic trees in the sense that each line of descent independently evolved to a level of organization that could be called procaryotic. The two lines of descent, nevertheless shared a common ancestor, that was far simpler than the procaryote. This primitive entity is called a progenote, to recognize the possibility that it had not yet completed evolving the link between genotype and phenotype. This concept changes considerably the view one takes toward cellular evolution.