COMPETITION BETWEEN ERYTHROMYCIN AND VIRGINIAMYCIN FOR INVITRO BINDING TO THE LARGE RIBOSOMAL-SUBUNIT

When the S component of virginiamycin binds in vitro to the [Escherichia coli] 50 S ribosomal subunit, a change of fluorescence intensity proportional to the amount of complex formed occurs. Erythromycin competes with virginiamycin S for attachment to ribosomes, and removes previously bound virginiamycin S from its target, as revealed by spectrofluorimetric analysis. The 50 S subunits which are incubated with the M component of virginiamycin (50 S*) have an increased affinity for virginiamycin S (the association constants of virginiamycin S with ribosomes are 2.5 .cntdot. 106 M-1 in the absence of virginiamycin M, and 15 .cntdot. 106 M-1 in its presence). Erythromycin does not compete with virginiamycin S for attachment to 50 S* subunits, nor is it able to remove virginiamycin S previously bound to the 50 S* subunit. Virginiamycin M produces a change in ribosomes, which results in a tighter complex virginiamycin S-50 S* subunit. Such change does not require the presence of virginiamycin M, as shown by the observation that ribosomes to which labeled virginiamycin M is transiently linked bind virginiamycin S in a form that cannot be removed by erythromycin.