Saccharomyces cerevisiae has a TFIIS-related transcription elongation factor, originally called P37 (Sawadogo, M., Sentenac, A., and Fromageot, P. (1979) J. Biol. Chem. 255, 12-15; Nakanishi, T., Nakano, A., Nomura, K., Sekimizu, K., and Natori, S. (1992) J. Biol. Chem. 267, 13200-13204), which binds directly to RNA polymerase II and stimulates read-through of intrinsic blocks to elongation. To elucidate functional features of this protein:protein interaction, we tested the ability of several forms of RNA polymerase II to respond to either full-length or an amino-terminal truncation of TFIIS. The variants of the polymerase differed in the structure of the carboxyl-terminal domain of the largest subunit or lacked two of the smaller subunits. No differences in ability to recognize intrinsic blocks to elongation or to read through them in response to either form of TFIIS were detected among these variants. Furthermore, ternary complexes containing each variant form of RNA polymerase cleave the 3' end of the nascent transcripts in response to TFIIS, a reaction previously reported for mammalian and Drosophila TFIIS (Kassavetis, G. A., and Geiduschek, E. P. (1993) Science 259, 944-945) and likely to be important in TFIIS function. Thus the carboxyl-terminal domain of the largest subunit and subunits four and seven of the polymerase, required in vivo, are not required in vitro for recognition of intrinsic blocks to elongation, read-through in response to TFIIS, or TFIIS-stimulated cleavage of the nascent transcript.