In vitro transcription of ribosomal RNA was studied on the DNA of the transducing bacteriophage λrifd 18, which carries an rRNA transcription unit from Escherichia coli. rRNA synthesis was preferential at all polymerase DNA ratios tested, and at the optimal 0.3 weight ratio nearly 60% of the transcript was rRNA. At this ratio the principal product of transcription comigrated in acrylamide-agarose electrophoresis with authentic 30-S rRNA precursor synthesized in vivo. In the presence of rifampicin more than one equivalent of rRNA was synthesized, thus suggesting the existence of two initiation sites for rRNA on the phage DNA. Similar results were obtained on E. coli DNA. Preincubation with heparin virtually eliminated the transcription of rRNA, in sharp contrast with the results of similar experiments on bacterial DNA where rRNA genes were transcribed 4-5 times in the presence of heparin. The possible explanation of this difference between rRNA promotors on the phage and the bacterial DNA are discussed.
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