A wild-type strain of Staphylococcus aureus that inactivates the 4,6-glycosidically linked deoxystreptamine aminoglycoside antibiotics by a plasmid-mediated process was found to harbor two enzymes: an acetyltransferase of the AAC(6') type and a new phosphotranferase specific to the gentamicin components. The target of this last enzyme is the 2''-hydroxyl function of these antibiotics, since one inactivated compound is 2''-(O)-phosphorylsisomicin.

No abstract

A wild-type strain of Staphylococcus aureus , which inactivates a wide variety of aminoglycosides (except the gentamicin components), has been found to harbor a plasmid (RAp01) that mediates the biosynthesis of a nucleotidyltransferase. This enzyme modifies the 4′-hydroxy function of these antibiotics. The plasmid has been studied, the enzyme responsible for this resistance pattern has been isolated by affinity chromatography, and its kinetics and physicochemistry have been characterized. The target of this enzyme has also been located by demonstrating the structure of one inactivated compound, 4′-( O )-adenylyltobramycin.

RP 59500 is a combination of RP 57669 and RP 54476, which are semisynthetic water soluble derivatives of pristinamycin IA (PIA) and pristinamycin IIA (PIIA), respectively. Like their precursors, these molecules are bacteristatic in their own right. In association, they exert bactericidal activity against a variety of Gram-positive bacteria. Experiments involving the binding of these antibiotics to the target bacterial ribosome showed that both the binding sites and the mechanism of action of the components of RP 59500 are identical to those of the parent molecules. By affinity-labelling with a structural analogue of RP 57669, it was demonstrated that L24, a protein of the 70S ribosomal subunit, was specifically labelled. Experiments using radioactive N-ethylmaleimide to label proteins possessing a thiol residue, indicated that proteins L24, L10 and L11 are not only close to each other in the ribosomal structure, but are also adjacent (if not actually part of) the channel through which newly synthesized proteins are extruded. We propose that the mechanism of action of these compounds is to close or narrow the extrusion channel of these proteins, which could lead to their accumulation on the ribosome. We cannot exclude, of course, the possibility that this accumulation disturbs peptidyl-tRNA hydrolase (PHT) activity, thereby depleting free tRNAs within the cell and inhibiting protein synthesis.

6′‐N‐[14C]Acetyl‐tobramycin and [3H]tobramycin were synthesized and their binding to Escherichia coli ribosomes and ribosomal subunits studied using equilibrium dialysis. The 70‐S ribosome, as well as its 50‐S and 30‐S subunits, bound tightly to 6′‐N‐acetyl‐tobramycin. The binding of [3H]tobramycin to ribosomes was quite different. The 70‐S ribosome was observed to possess several classes of binding sites; of these, one was determined to be of higher affinity and lower capacity, the 6′‐N‐[14C]acetyl‐tobramycin site. The isotopic dilution method was used to define the specificity of the interaction. The selective binding of 6′‐N‐[14C]acetyl‐tobramycin was highly reversible by tobramycin, kanamycins A, B, C and neomycin, but not by streptomycin or erythromycin. Gentamicin C1a was a poor inhibitor. This suggested that either the kanosamin or garosamin rings might be determinant in the binding of these molecules, as well as the 6′‐amino group.