SUMMARY: Of 144 antibiotics examined with respect to their action on Euglena chloroplasts, 46 caused irreversible loss of plastids and most inhibited chlorophyll synthesis. These substances included structurally related compounds as well as degradation products of antibiotics. Antibiotics exhibiting bleaching activity were of two general types judged by their mechanisms of action in other systems:

1. Inhibitors of DNA synthesis – anthramycin, edeine, porfiromycin, some mitomycins, myxin, nalidixic acid and its derivatives, novobiocin, primycin, rubiflavin, sarkomycin and streptonigrin;

2. Inhibitors of protein synthesis – 29 antibiotics which carry a common molecular denominator in their structure (an aminohexose) and three antibiotics which lack aminosugar moieties: viomycin, streptogramin and pactamycin.

Only these two types of antibiotics permanently eliminated chloroplasts; antibiotics classified as possessing other mechanisms of action were not effective.

All these bleaching antibiotics inhibited replication of plastids in concentrations having no effect on normal Euglena division. A diluting-out of pathological plastids is the explanation of this ‘bleaching phenomenon’.


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