Residual RNA Synthesis in Escherichia coli after Inhibition of Initiation of Transcription by Rifampicin

Recent studies on the mechanism of action of rifampicin have shown that the drug inhibits the initiation of transcription by interacting with RNA polymerase without preventing the completion of RNA chains in the process of being synthesized (see lVIosteller and Yanofsky, 1970, for a survey of the relevant literature). A technique has been developed to use the properties of this drug to study the distribution of polymerase molecules in bacterial cells and related problems concerning RNA synthesis. THE WAVE OF RESID~.TAL RNA SY:NTHESIS The incorporation of 3H-uridine into acid prccipitable material has been followed after addition of label and rifampicin to an exponen-tially growing culture of AS 19, a rifampicin sensitive strain of E. cell B (Sekiguchi and Iida, 1967). Figure 1 shows the labeling pattern obtained with and without the further addition of nalidixic acid to prevent label entering into DNA. The curve obtained by using both drugs can be generated from the broken curve (rifampicin alone) by subtracting from each value the counts remaining after alkaline hydrolysis. This indicates that nalidixic acid blocks DNA synthesis without interfering with the residual synthesis of RNA. In the presence of both drugs, 3H-uridine is initially incorporated at a decreasing rate and the incorporation reaches amaximum at about 1.5 rain; subsequently part of the labeled material decays, and after 10 min an almost constant level is reached. The slight increase commonly seen after 10 rain is paralleled by an increase observed in control cultures to which label is added 5 rain after rifampicin. The curve of residual RNA synthesis after addition of rifampicin is complex and contains information about several aspects of RNA syn-thesis: (a) The most evident feature is the appearance of label in an unstable as well as a stable RNA fraction. We identify the unstable fraction (a) with messenger RNA (mRNA) and the stable fraotion (b) with the ribosomal RNA (rRNA) and transfer RNA (tRNA) species. The ratio Q = a/(a + b) is used as an estimate of th