The isolated subunits of Escherichia coli DNA-dependent RNA polymerase are reassembled in a stepwise manner in the following sequence: 2 alpha leads to alpha 2 leads to alpha 2 beta leads to alpha 2 beta beta' (premature core enzyme) leads to E (active core enzyme). When the in vitro reconstitution is performed at low temperature, the subunit assembly is prevented until the assembled but inactive premature core enzyme is formed, which is similar to native core enzyme in many parameters of gross conformation but differs from it in several minor and local conformations. The temperature-dependent activation of premature core enzyme at a salt concentration as low as that in vivo takes place only in the continuous presence of either the sigma subunit or DNA. The sigma subunit is therefore proposed to be a regulatory protein which influences the conformation of core subunit assembly in multiple ways from the initial enzyme maturation to the final initiation of transcription. Evidence has accumulated which indicates that the subunit assembly in vivo proceeds via the same pathway as that identified in vitro, including the identification of all species of the assembly intermediates in cell extracts, the identification of all possible types of assembly-defective mutants among temperature-sensitive alpha-, beta-, and beta'-subunit mutants, the kinetics of the appearance of pulse-labeled subunits in the enzyme structure as expected from the assembly sequence and the integration of labeled subassemblies into the enzyme structure upon chasing. The functional complexity of RNA polymerase coupled with transcriptional control appears to depend on its structural flexibility which fluctuates through the assembly with various transcription factors. This type of transcriptional control is being thoroughly considered by a final conclusion awaits further examinations.