Molecular mechanism of action of virginiamycin-like antibiotics (synergimycins) on protein synthesis in bacterial cell-free systems.

Synergimycins contain two types of components, A and B, which synergistically inhibit in-vivo protein synthesis. In-vitro, both components interact with 50 s ribosomal subunits. B components bind in stoichiometric amounts (KA = 2.5 X 10(6) M-1): the association constant undergoes a ten-fold increase in the presence of A components. The latter inhibitors act in substoichiometric concentration (KA = 0.32 X 10(6) M-1) and produce lasting ribosome damage due to a conformational alteration requiring proteins L7/L12, L8 and L16. Such alteration entails a permanent block of the peptidyltransferase substrate acceptor site, whereby aminoacyl tRNA enzymically bound to ribosomes is released. Macrolides (KA = 7.2 X 10(7) M-1) displace ribosome-bound B components, but they are unable to compete in the presence of A components, which reduce both the affinity of ribosomes for macrolides, and the dissociation rate constant of B components. These findings provide a molecular explanation to the synergistic action of A and B components in-vivo.