Structure and regulatory properties of eucaryotic RNA polymerase

It has been frequently proposed and popularly held that selective gene transcription i eucaryotes is a result primarily of the specificity of the tem-plate itself. According to this view, the activity of a particular gene is regulated by the macro-molecules associated with the DNA in the chro-matin matrix (Stedman and Stedman, 1950; Bonner et al., 1968; Davidson, 1968). It is implied that the specificity of the I~NA polymerase is restricted to the recognition of the nucleotide substrates and that there is a single generalized initiation site on the DNA. Transcription then occurs on those cistrons which are not repressed or sequestered. Perhaps the most persuasive evidence supporting this view is the demonstration of apparent transcriptive specificity from chromatin using bacterial RNA polymerase as the transcrip-tive agent (Paul and Gilmour, 1968; Smith et al., 1969). An alternative possibility involves a number of different sites on the chromosome for the initiation of transcription. These sites might be discriminated by multiple RNA polymerases with different initiator site specificities or by a single polymerase with multiple regulator subunits that prescribe read-out specificity. This latter mechanism seems attractive for regulating the activity of at least certain major classes of cistrons for the following reasons. (1) Procaryotic hromosomes apparently do not have a complex chromosomal matrix, yet these systems exhibit transcriptive specificity. Furthermore, Burgess et al. (1969) have discovered factors which interact with E. coli RNA polym-erase and confer initiator specificity on the "core" enzyme. (2) Multiple forms of many enzymes have been described, especially in eucaryotic systems, and in some instances pecific physiological roles for individual species have been demonstrated. (3) The ribosomal genes are apparently independentl