Constitutive Transcriptional Control Signals of the Herpes Simplex Virus tk Gene

Most of the molecular details of eukaryotic transcrip-tional regulation are poorly understood. Two important facts, however, have been established. First, a eukaryo-tic gene can exist either in a transcriptionally active state or in a repressed state. This state of activity is dictated by as yet ill-defined temporal and tissue-specific con-siderations imposed by the ontogeny of the organism. 1 Second, an active eukaryotic gene is transcribed at a specific efficiency that represents an inherent property of that gene. 2These rather general observations lead one to view the process of transcription i itiation as a two-component system. One component dictates whether or not a gene is actively expressed. This first component can be compared with the "on/off switch " of an electric blender. A second component involved in regulating