Mediator Complex Subunit 21 (MED21)

Transcription of class II genes in S. cerevisiae cells is initiated by a large multisubunit complex containing RNA polymerase II (RNAPII), general transcription factors, and SRB (suppressor of RNA polymerase B) proteins. By searching an EST database for sequences similar to yeast Srb7, Chao et al. (1996) identified partial human SRB7 cDNAs. They used PCR with primers based on the sequences of the partial cDNAs to isolate a lymphocyte cDNA corresponding to the entire SRB7 coding region. The predicted 144-amino acid human protein is 35% identical to yeast Srb7. Although the full-length human cDNA failed to complement a yeast Srb7 deletion, several chimeric cDNAs encoding the N terminus of the human protein and the C terminus of the yeast protein were able to do so.