Unregulated transcription of protein-encoding genes in vitro is dependent on 12-subunit core RNA polymerase II and five general transcription factors; TATA binding protein (TBP), transcription factor (TF)IIB, TFIIE, TFIIF, and TFIIH. Here we describe cloning of the mouse cDNAs encoding TFIIB and the small and large TFIIE and TFIIF subunits. The cDNAs have been used to express the corresponding proteins in recombinant form in Escherichia coli and in Sf21 insect cells, and all proteins have been purified to . 90% homogeneity. We have also purified a recombinant His 6 -tagged mouse TBP to near homogeneity and show that it is active in both a reconstituted mouse in vitro transcription system and a TBP-dependent in vitro transcription system from Saccharomyces cerevisiae. The more complex general transcription factors, TFIIH and RNA polymerase II, were purified more than 1000-fold and to near homogeneity, respectively, from tissue cultured mouse cells. When combined, the purified factors were sufficient to initiate transcription from different promoters in vitro. Functional studies of the S-phase-specific mouse ribonucleotide reductase R2 promoter using both the highly purified system described here (a mouse cell nuclear extract in vitro transcription system) and in vivo R2-promoter reporter gene assays together identify an NF-Y interacting promoter proximal CCAAT-box as being essential for high-level expression from the R2 promoter.Keywords: in vitro transcription; RNA polymerase II; ribonucleotide reductase; mouse general transcription factors.RNA polymerase II catalyses transcription of all proteinencoding genes in eukaryotic cells. The polymerase is dependent on accessory proteins, called the general transcription factors (GTFs) for accurate initiation in vitro [1][2][3][4]. One of these factors, TATA binding protein (TBP), is able to bind directly to promoter DNA and to initiate the sequential addition of the remaining components of the transcription initiation complex. The minimal set of GTFs required for basal transcription in vitro includes, in addition to the RNA polymerase and TBP, also TFIIB, TFIIE, TFIIF, and TFIIH. These factors are both necessary and sufficient for proper initiation of transcription in in vitro transcription systems purified from yeast Saccharomyces cerevisiae, rat and human cells [2,[5][6][7][8].The identification of a minimal set of GTFs, cloning of the corresponding cDNAs from different species and the recently described three-dimensional structure of the yeast RNA polymerase II [9] have facilitated studies of the factors involved in forming a functional preinitiation complex. Swapping experiments of GTFs purified from the distantly related yeasts S. cerevisiae and Schizosaccharomyces pombe have shown that none of the GTFs, except TBP, is individually exchangeable between the different systems [10]. However, the combined substitutions of S. pombe TFIIB and RNA polymerase or TFIIE together with TFIIH, with the corresponding S. cerevisiae factors are possible and thus indicate a specie...