Determination of 5′-triphosphate terminus of 16S ribosomal RNA precursor

Abstract

THE genes for the 23S and 16S rRNAs are physically linked¹. Using the drugs rifampicin and actinomycin D, it has been shown that the 23S and 16S rRNA cistrons are co-transcribed and the promotor is proximal to 16S rRNA cistron^2,3. Recent work with an Escherichia coli strain deficient in RNase III has provided further evidence for co-transcription, showing the presence of a long polycistronic RNA which could be cleaved to the immediate precursors of 23S and 16S rRNAs when treated with RNase III in vitro⁴. We have now studied the transcription of rRNA in E. coli using γ-³²P-labelled triphosphates and have shown that the 16S rRNA precursor in our preparation has a 5′-pppA end and that in our system it is neither cleaved nor replaced by 5′-pA as observed by Takanami⁵. We have not found any detectable 5′-γ-³²P-triphosphate end in the 23S rRNA species. This observation further verifies directly that both the 16S and 23S rRNAs belong to one transcriptional unit in the following order: promotor–16S–23S.