An RNA polymerase II transcription system was resolved and reconstituted from extracts of Schizosaccharomyces pombe. Exchange with components of a Saccharomyces cerevisiae visiae system was undertaken to reveal the factor or factors responsible for the difference in location of the transcription start site, about 30 base pairs and 40 to 120 base pairs downstream of the TATA box in S. pombe and S. cerevisiae, respectively. Two components, counterparts of human transcription factor IIF (TFIIF) and TFIIH, could be exchanged individually between systems without effect on the start site. Three components, counterparts of human TFIIB, TFIIE, and RNA polymerase II, could not be exchanged individually but Gould be swapped in the pairs TFIIE-TFIIH and TFIIB-RNA polymerase II, which demonstrates that there are functional interactions between these components. Moreover, exchange of the latter pair shifted the starting position, which shows that TFIIB and RNA polymerase II are solely responsible for determining the start site of transcription.