The RNA polymerase is structurally one of the most complex enzymes in the bacterial cells. This key component of the machinery for the expression of genetic information is a heteromultimer of molecular weight of 5×105 daltons and composed of at least four nonidentical subunits. The overall transcription activity is determined bythe concentration of RNA polymerase as well as the concentration of promoter sequences in DNA. In this context the intracellular level of RNA polymerase is potentially a rate limiting factor in cell growth. It has been established that, though the overall concentration of DNA stays virtually constant, the concentration of promoters available for interaction with RNA polymerase changes depending upon the concentration of repressors. In contrast, little is known how the intracellular levels of RNA polymerase is maintained under different conditions of cell growth and how those levels relate to the physiological requirements of the cells. Strong insights have been provided recently into the control of RNA polymerase formation, which include the regulation of the expression of the genes encoding the individual subunits of polymerase, and of their assembly into functional polymerase. These studies suggest that the level of RNA polymerase controls the expression of polymerase genes. The molecular mechanism of this auto-regulation, however, remains to be investigated, including whether this is at the transcriptional or translational level and what is the effector, a subunit of polymerase, a product of polymerase or polymerase itself. These lines of the studies may also extend to the regulation of the transcription specificity of RNA polymerase through interaction with intracellular metabolites, whose intracellular level is tightly coupled to the state of cell growth. In this special issue, the following articles are included: I. Control of RNA polymerase formation (1) Nakamura, Y. (Institute for Virus Research, Kyoto University): The genetics of RNA polymerase in Escherichia coli. (2) Taketo, M. (Institute for Virus Research, Kyoto University): Formation of RNA polymerase in Escherichia coli. (3) Tsuji, S., Imahori, K. (School of Medicine, University of Tokyo): DNA-dependent RNA polymerase from T. thermophilus HB8. Crystal structure and reconstitution from isolated subunits. (4) Yamamoto, M. (Institute of Enzyme Research, University of Wisconsin): The relationship between the synthesis of RNA polymerase and that of ribosome. II. Control of transcription specificity of RNA polymerase (1) Fujiki, H. (National Cancer Research Institute (Tokyo)): Structural Modifications and Regulational changes of DNA dependent RNA polymerase from E. coli. (2) Muto, A. (Research Institute for Nuclear Medicine and Biology, Hiroshima University): Conversion of transcriptional specificity of RNA polymerase.