High level expression in E. coli and purification of yeast transcription factor IIIA.

Saccharomyces cerevisiae transcription factor IIIA, a sequence-specific DNA binding protein that is required for transcription of 5S rRNA genes by RNA polymerase III, has been expressed in Escherichia coli in a full length, native form. High level expression was achieved through the combined use of a T7 RNA polymerase expression system and of a multicopy plasmid carrying an E. coli gene, argU, which codes for a minor Arg(AGA/AGG) tRNA species. Recombinant yeast transcription factor IIIA was purified to 95% homogeneity, at a final yield of 8 mg/liter of bacterial culture, by three chromatographic steps, and it was shown to be at least 55% active by quantitative in vitro transcription assays.