Structure and transcription specificity of yeast RNA polymerase A.

DNA-dependent RNA polymerase A (Nucleosidetriphosphate: RNA nucleotidyltransferase, EC 2.7.7.6) was isolated from whole yeast cells and purified to a nearly homogeneous state. The subunit structure as well as the transcription specificity of the purified enzyme were investigated. Polyacrylamide gel electrophoresis under denaturating conditions revealed that yeast polymerase A is made up of two large subunits having mol. wts of 190 000 and 135 000, and five smaller subunits with mol. wts of 54 000, 44 000, 35 000, 25 000 and 16 000, respectively. The molar ratios of all these polypeptides were found to be about unity. The transcription specificity of yeast polymerase A was tested using homologous nuclear DNA as a template. The in vitro synthesized RNA was characterized by determining its degree of self-complementarity and its ability to compete with purified ribosomal RNA in hybridization experiments. It was found that yeast polymerase A is capable of a highly selective transcription in vitro of the rRNA cistrons, provided DNA of high integrity is used as a template.