Labile 50S ribosome from partial macrolide-resistant Staphylococcus aureus.

Staphylococcus aureus S704 and 8325MMT7 show constitutive resistance to macrolide antibiotics such as erythromycin (EM), oleandomycin, spiramycin, rosamicin and josamycin, except for tylosin, rokitamycin (RKM), and mycinamicin as well as lincosamide and streptogramin type B antibiotics (PM-resistance). Whenever 70S ribosomes from either of them were dissociated into 30S and 50S subunits in a 10-28%(W/W) linear sucrose gradient, the latter subunit was further cleaved into two small apparently equal particles (about 40S). RKM could no longer bind to either of the small particles. A prior exposure of 8325MMT7's 50S subunit to RKM (except EM) did not cause cleavage in any small particles. The largest component (M.W. 33.0kDa) of 50S ribosomal proteins was absent in at least the small particles. The first finding suggests that the lability of the 50S ribosome may be responsible for PM-resistance.