THE synthesis of ribosomal RNA (rRNA) in exponentially growing bacteria accounts for approximately half of the total RNA synthesis, although the ribosomal DNA (rDNA) genes make up only 0.3-0.4% of the genome1. Electronmicroscopic observations2 and biochemical data3,4 suggest that the frequency of transcription initiation on these genes is much higher than on any other part of the bacterial chromosome. If the same enzyme is responsible for the transcription of all kinds of bacterial RNA, this difference in the initiation frequency might be due either to a specific regulatory factor or to the structure of the relevant interacting macromolecules themselves. If the latter were true, the DNA promoter sequence responsible for the initiation of transcription on rRNA genes should be more active than other promoters. This hypothesis is supported by the discovery of preferential in vitro rRNA synthesis with purified RNA polymerase in the absence of any external factor5-7.
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