QUINUPRISTIN (RP-57669) - A NEW TOOL TO INVESTIGATE RIBOSOME-GROUP-B STREPTOGRAMIN INTERACTIONS

Streptogramin antibiotics consist of two types of molecules, group A a nd 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), Wh en group B molecules bind to 50S subunits or to tightly coupled riboso mes, there is an increase in their fluorescence intensity, which is pr oportional to the concentration of the antibiotic-ribosome complex for med. We found here that the background fluorescence of unbound quinupr istin 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 molec ules to tightly coupled 70S ribosomes in the absence of the group A mo lecules (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 th e corresponding group A molecule (quinupristin + dalfopristin: 69 x 10 (7) M-1; virginiamycin S + virginiamycin M: 60 x 107 M-1). Similar res ults were obtained with 50S ribosomal subunits. Additionally, we provi de evidence that the failure of the group B molecules to inhibit poly( Phe) synthesis is due to the displacement of the group B molecule duri ng poly(Phe) polymerization on the ribosome, indicating that the artif icial poly(Phe) peptide competes with the binding of the group B molec ule.