Quinupristin (RP 57669): a new tool to investigate ribosome-group B streptogramin interactions

Streptogramin antibiotics consist of two types of molecules, group A and group B. The group B molecule quinupristin (RP 57669) and the group A molecule dalfopristin (RP 54476) constitute the first water-soluble semisynthetic streptogramin, quinupristin/dalfopristin (RP 59500). When group B molecules bind to 50S subunits or to tightly coupled ribosomes, there is an increase in their fluorescence intensity, which is proportional to the concentration of the antibiotic-ribosome complex formed. We found here that the background fluorescence of unbound quinupristin is 10-fold lower than that of unbound virginiamycin S, a natural group B molecule often used experimentally. The association constants were found (i) to be similar for the binding of the two group B molecules to tightly coupled 70S ribosomes in the absence of the group A molecules (quinupristin: 3.5 x 10(7) M(-1); virginiamycin S: 2.8 x 10(7) M(-1)) and (ii) to similarly increase about 20-fold in the presence of the corresponding group A molecule (quinupristin + dalfopristin: 69 x 10(7) M(-1); virginiamycin S + virginiamycin M: 60 x 10(7) M(-1)). Similar results were obtained with 50S ribosomal subunits. Additionally, we provide evidence that the failure of the group B molecules to inhibit poly(Phe) synthesis is due to the displacement of the group B molecule during poly(Phe) polymerization on the ribosome, indicating that the artificial poly(Phe) peptide competes with the binding of the group B molecule.