Multiple modes of ribosomal RNA transcription in vitro

The rate of rRNA synthesis from E. coli DNA in vitro can exhibit an unusual temperature dependence. Instead of the typical sigmoid curve, at least five to six pronounced peaks of rRNA synthesis are detectable over a 30° range in temperature. rRNA synthesis from preformed initiation complexes exhibits a similar pattern and shows that the number of polymerases able to initiate rRNA synthesis increases with each successive peak. These results are interpreted in terms of a model which proposes that the rRNA cistrons are served by multiple promoter sites (subpromoters) whose activation energies form a graded series. Thus the number of polymerases initiating rRNA synthesis could be controlled by regulating the number of active subpromoters.