The multisubunit general transcription factor termed TFIID is comprised of the TATA box DNA binding protein TBP and several TBP-associated factors termed TAFs. Current arguments regarding the mechanisms of regulation of transcription contend that TFIID makes multiple specific protein-protein interactions with numerous protein factors, and that these interactions are important for the regulation of transcriptional initiation. TAFs contain a variety of potential structural motifs and it has been speculated that these motifs participate directly in TAF function. However, to date the physiological significance of these putative structural motifs has not been systematically analysed in vivo. The essential gene encoding the Schizosaccharomyces pombe 72 kDa TFIID subunit is termed taf72+, which contains WD40 repeats, was cloned and sequenced. A comparison of the primary structure of this gene with its Drosophila and S. cerevisiae counterparts suggests the presence of regions that might play a role in TFIID function, due to the fact that significant portions of the sequences are highly conserved. Complementation analyses of a series of deletion mutants of this gene revealed that the most evolutionarily conserved regions of taf72+, including the WD40 repeats, are in fact indispensable for the viability. The 72 kDa subunit of S. pombe TFIID, which contains putative WD40 repeats, consists of three distinct functional domains separated by intervening regions. The functional significance of the WD40 repeats is demonstrated by this in vivo study.