Abstract
The M component of virginiamycin inhibited growth of Plectonema boryanum under both photoautotrophic and heterotrophic conditions. Though the S component of this antibiotic had no apparent activity per se, it enhanced the inhibitory action of its partner. Cells incubated with suitable concentrations of either M or M + S stopped growing and lysed. Loss of the colony-forming capacity occurred quickly in the presence of M + S and slowly in the presence of M alone. Virginiamycin M inhibited protein synthesis in autotrophically and heterotrophically growing Plectonema. This effect was very rapid and could be reversed by removing the antibiotic. The S component did not block the incorporation of amino acids into proteins, but prevented the reversibility of the inhibitory effect of M. Virginiamycin M or S did not affect the photosynthetic oxygen development (Hill's reaction) in Plectonema. Moreover, carbon dioxide photoassimilation and formation of chlorophyll were inhibited only after an appreciable lag. Deoxyribonucleic acid synthesis was blocked virtually without delay by virginiamycin M. Since virginiamycin inhibited protein synthesis in a similar fashion in the unicellular Anacystis nidulans, as well as in the filamentous P. boryanum, the mechanism of action of this antibiotic is probably the same in all blue-green algae.