Antibiotic Receptor-Sites in Escherichia coli Ribosomes

Identification of the 30-S ribosomal proteins at the decoding site by affinity labelling with a reactive oligonucleotide

Proteins S18 and S1 were identified by immunological and electrophoretic techniques as constituents of the 30-S ribosomal decoding site by the use of G-U-U-i[3H]acn5U (where i[3H]acn5U=5-iodo[3H]acetylaminouridine) as a chemical affinity label. S12 was modified to a minor extent and mainly when assays were carried out in the absence of tRNA. 1. Evidence for the specificity of the covalent binding of the oligonucleotide probe is given by the following experiments: G-U-U-iacn5U bound twice as much Val-tRNA as compared to G-U-U-U. The covalent binding was strongly inhibited by prebound poly(U)-Phe-tRNA. 'Programmed' ribosomes, i.e. ribosomes which contained only covalent bound oligonucleotide, stimulated the binding of Val-tRNA. 2. Val-tRNA bound to G-U-U-iacn5U labelled ribosomes could not be transferred to puromycin. This might indicate that binding of the oligonucleotide occurred at the aminoacyl site. 3. 80-90% of the radioactivity was found in the 30-S subunit and only the protein moiety of the ribosome was modified. 4. Unambiguous identification of the labelled proteins was achieved by immunological techniques and revealed proteins S1 and S18 as constituents of the 30-S ribosomal decoding site.

Parameters of acquired resistance and their role in the evaluation of new chemotherapeutic drugs

Acquired resistance can be defined as a qualitative alteration of the genetic material of a cell which is phenotypically correlated with a measurable decrease of the cell's sensitivity against one or several chemotherapeutic agents. There are two basic genetic mechanisms which can lead to the emergence of resistance: mutation and the acquisition of additional genetic material from another cell. Both forms of resistance play an important role in clinical situations: the emergence of resistance by mutation occurs in tumor cells and can also lead to therapeutic problems in antimicrobial chemotherapy. In bacteria, however, acquisition of resistance plasmids represents the dominating mechanism which is responsible for most therapeutic problems in the clinical environment. The different genetic mechanisms involved in the emergence of resistance are paralleled -- at least in bacteria -- by two principally different groups of biochemical mechanisms implementing resistance. Mutations lead to alterations of single cell constituents such as the cell membrane or cellular receptors necessary for the binding of the antimicrobial agent. This form of resistance is biochemically characterized by the inaccessibility of the cell interior for a particular compound or by the modification of an intracellular binding site which loses its affinity for the chemotherapeutic agent. Resistance plasmids on the other hand code for enzymes which inactivate the antibiotic (beta-lactamases, aminoglycosideinactivating enzymes, chloramphenicol-acetyltransferase); In some cases, they direct the synthesis of proteins which affect cell permeability (tetracycline) or isoenzymes which have a lower affinity for the inhibitor (trimethoprim). Resistance against antibiotics can be inducible; In these cases the regulatory mechanisms involved are stable genetical traits as resistance itself; Using chloramphenicol, beta-lactam-antibiotics and aminoglycosides as examples, it is demonstrated that resistance data gathered early in the development of a new drug are of little value in estimating the clinical potential of a new compound. Information on the rate at which resistance develops, on the pattern according to which it emerges ("single step" or "multi step") and on cross-resistance patterns is important in the characterization of a new drug but is often invalidated by later findings obtained in the clinical environment; The problem appears somewhat simpler if a new drug is a member of an already known class of compounds, e.g. a beta-lactam or an aminoglycoside. In such cases our knowledge of frequent enzymatic inactivation mechanisms provides a basis not only for the evaluation of an existing drug, but also for the synthesis of new derivatives.