The effects of three antibiotics on the synthesis of the small‐molecular‐weight nuclear RNA (called small nRNA) have been investigated. These antibiotics have been chosen for their different mechanism of action on RNA synthesis. From these investigations the following observations can be made. a) Low concentrations of actinomycin D, whose effect on the synthesis of nuclear‐precursor ribosomal RNA is well known, abolish the synthesis of the 7‐S rRNA or E peak (or 28‐S‐associated rRNA). Amounts ten times higher are not sufficient to inhibit completely the synthesis of other small nRNAs although the A, C and D peaks and the methylated species of the G and G' peaks are much more inhibited than the 5‐S rRNA. b) Toyocamycin also depresses the synthesis of the 7‐S rRNA or E peak. It does not inhibit however the synthesis of the other nucleoplasmic peaks such as the C, D and G‐G' species. It seems to exert an indirect effect on the nucleolar A, B and H species of small nRNAs at the same time as it blocks the conversion of the 45‐S precursor rRNA into the mature 28‐S and 18‐S rRNA. c) The mechanism of action of cycloheximide on small nRNAs synthesis was investigated both in monolayers and suspension cultures. Although protein synthesis was completely inhibited in both cases, effects on small nRNAs were different: a much greater degradation of various classes of RNA was observed when suspension cultures were used. It is suggested that this degradation was due to an instability of some species of newly synthesized RNA upon cycloheximide treatment.
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Hamelin et al. (1973) studied this question.