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Protein synthesis is accomplished through a process known as translation and is carried out by the ribosome, a large macromolecular complex found in every living organism. Given the huge amount of biological data that must be deciphered, it is not uncommon for ribosomes to regularly stall during the process of translation. Any disruption of this finely tuned process will jeopardize the viability of the cell. In bacteria, the main quality-control mechanism for rescuing ribosomes that undergo arrest during translation is trans-translation, which is performed by transfer-messenger RNA (tmRNA) in association with small protein B (SmPB). However, other rescue systems have been discovered recently, revealing a far more complicated network of factors dedicated to ribosome rescue. These discoveries make it possible to consider inhibition of these pathways as a very promising target for the discovery of new antibiotics., (© 2015 médecine/sciences – Inserm.)

Rapamycin (sirolimus) is a macrolide, related to cyclosporine with immunosuppressive properties and antiproliferative activity in various human tumor cells lines and tumor xenograft models. The cytosolic kinase mTOR which controls the initiation of the translation of messenger RNA is the main known target of rapamycin. During clinical studies, rapamycin given by oral route as immunosuppressant did not show dose-limited toxicity and only asymptomatic thrombopenia and hyperlipemia were observed. In murine models, best antitumoral activity was observed using parental routes. CCI-779, an analog formulated for intravenous use has antitumor activity without significant immunosuppressive property in mice and is currently in phase I trials in man.

The selective inhibitor of ribosomal RNA synthesis toyocamycin, abolishes at very low concentrations, the erythroid differentiation of Friend cells when used under conditions where other inhibitors of DNA and RNA synthesis do not impede differentiation. Toyocamycin does not inhibit the synthesis of 16 S premessager RNA for globin, nor does it prevent splicing for mature 9 S globin messenger. Inhibition of differentiation might be explained by an effect at the translation level.

The occurence of a polypeptidic synthesis during the in vitro cytotoxic reaction of sipunculid worns is demonstrated. Some drugs such as actinomycin D, cycloheximide, cytochalasin B, Colchicine, vinblastine and iodoacetate have been tested. Hypothesis are expressed on the use of the products synthetized by leukocytes.

The present paper is an attempt to review the hypotheses concerning the mechanism of the physiopathological effects induced by mycotoxins, in particular by the aflatoxins and PR toxin. We shall first examine to what extent the alterations produced on some metabolic pathways (transcription, translation) can explain the short terme toxic effects. The problem of the long term effects of aflatoxin B1, i.e. the hepatocarcinogenic effects, will be discussed taking into account the results concerning the action of the mycotoxin on DNA and DNA repair.

Patulin, a mycotoxin isolated from cultures of Byssochlamis nivea, alters the transcriptional and translational processes. Using cell-free systems derived from procaryotic and mammalian cells, we have studied the mechanism of the mycotoxin-induced inhibition of in vitro RNA and protein syntheses.

The specificity of phenylalanine and tyrosine carriers was investigated using actively metabolizing cells of Brevibacterium linens. The cellular protein synthesis of resting cells was very weakly inhibited, even with high concentrations of chloramphenicol or tetracycline. The nonaromatic amino acids were weak inhibitors for these carriers, while fluorinate analogues of phenylalanine and tyrosine were very potent competitive inhibitors. In practice these analogues cannot be used to replace amino acids to evaluate transport without incorporation because they are incorporated in cellular proteins.

Inhibitors of protein synthesis caN elicit the precocious appearance of a grey crescent (GC) in in vitro maturing Ambystoma mexicanum oocytes. This treatment however fails to induce GC formation when the oocytes are enucleated before initiation of maturation. The ability to form a GC is reestablished in enucleated oocytes by the injection of nucleoplasm from a normal oocyte, either before or after injection of the inhibitor. In the latter case, the GC appears even though the protein synthesis level is already about one-tenth that of the control enucleated oocytes. One or several nuclear factors, in conjunction with protein synthesis inhibition, are therefore essential for the early symmetry reaction. The corrective nuclear factor is already present in the germinal vesicle of young oocytes at the very beginning of vitellogenesis. It is not species-specific and enucleated axolotl oocytes can be symmetrized with Pleurodeles or even Xenopus oocyte nucleoplasm. The interaction between nuclear factor(s) and cytoplasm is possible only when cytoplasmic maturation has occurred.

Polysome level was measured in Escherichia coli cells grown in the presence of sublethal doses of four different aminoglycosides which all induce mistranslation: streptomycin, neomycin, kanamycin and gentamycin. No significant variation was noticed as referred to the level of polysomes in normally growing cells. This observation indicates that the erroneous incorporation of amino acids into polypeptide chains does not necessarily result in the release of ribosomes from messenger RNA molecules.

A dialysable and thermostable compound was partially purified from Cnestis glabra (Connaracea) seeds. It is insoluble in organic solvents. At a dose corresponding to 0.5 g of seeds it has a lethal effect on mice after convulsive attacks. Sodium dipropylacetate, an anticonvulsive agent prevents the convulsions but not the death of the animals. The toxic compound inhibits protein synthesis in a rabbit reticulocyte lysate and in hepatoma tissue culture; moreover in the cell culture, the incorporation of thymidine into DNA is inhibited.

We examined the effect of diabetes on protein synthesis in intestinal epithelial cells. Isolated enterocytes of diabetic rats display a marked decrease (50%) in 3H-leucine incorporation into intestinal proteins as compared to normal cells. Treatment of diabetics with insulin restores the level of protein synthesis up to normal values. Similarly, in vitro translation products of isolated RNA are much lower in diabetics than in normal or treated diabetics. It is concluded that in intestinal cells, insulin can play an important role in regulating the biosynthesis of proteins by modulating the translation of specific RNAs.

Antibiotics are very commonly used substances to eradicate bacterial infections by bacteriostatic or even bactericid effect. They act at a very specific stage (target), although other less important or secondary interactions can occur. We studied the interaction of three antibiotic families (beta-lactamins, aminosides, rifampicin) with bacterial cell. Penicillin disturbs the cell wall synthesis and more accurately the glycopeptide (or murein) formation, a substance giving rigidity or shape to bacteria. It acts in the late phase of murein-biosynthesis, when N-acetyl glucosamin -- N-acetyl muramic acid L ala -D glu M-DAP (L lys) -D ala -D ala are linked together by the peptide part, under the effect of several enzymes, particularly transpeptidase and DD-carboxy-peptidase. It would appear that beta-lactame-thiazolidine rings have a steric analogy with dipeptide D-alanyl D-alanine. The result would be that the enzyme would act on the antibiotic instead of peptide: the consequence would be inhibition of the peptidic link, giving an abnormal murein, and an incomplete cell wall i.e. fragile bacteria. Aminosides, particularly Streptomycin, link themselves to 30 S subunit of bacterial ribosome. In this case, it seems that it is a 3''OH function which reacts with lysine (from S 12 protein part of 30 S subunit). The consequence is an alteration in the RNA messager lecture, and a false traduction and consequently protein biosynthesis stops with a decrease of polyribosomes and of the formation of inert 70 S ribosome. Rifamycins, and particularly Rifampicin act by inhibition of RNA messager synthesis. One molecule of antibiotic links itself to one molecule of RNA messager : hydroxyl and cetone function in C1 Cs C21 C23 and "ansa" bridge link to beta subunit of RNA polymerase. This linkage gives a conformational change to the RNA polymerase-DNA complex, inhibiting the catalytic action of this enzyme, and consequently stopping RNA messager and protein synthesis. The study of the action mechanism of these antibiotics enables us to show the action specificity of these products in the bacteria. This specificity is more accurate when the target is not to be found in the eucaryotic cells : in this case the antibiotic may be considered as entirely atoxic. If the study of the action mechanism of antibiotics gives a better understanding of the use of these drugs, their action at a definite stage in bacterial metabolism is a valuable tool for scientists in their approach to cell functioning.