Copyright information:Taken from "Tfb5 interacts with Tfb2 and facilitates nucleotide excision repair in yeast"Nucleic Acids Research 2007;35(3):861-871.Published online 10 Jan 2007PMCID:PMC1807977.© 2007 The Author(s) A ring structure is formed by Ssl1, Rad3 and Ssl2, on which another layer of ring structure is formed by Tfb1, Tfb2 and Tfb4 through Ssl1–Tfb1, Ssl1–Tfb4 and Ssl2–Tfb2 interactions. However, Tfb1, Tfb2 and Tfb4 do not interact with each other, resulting in a fragile layer of ring structure. This fragile 6 subunit core TFIIH is stabilized by fitting the small Tfb5 into the structure through the Tfb5–Tfb2 interaction. Tfb5 may also help locking Tfb1 in place in the core TFIIH ring structure. Thus, the Tfb5 protein uniquely serves as the architectural stabilizer of the core TFIIH. The TFIIK sub-complex consisting of Tfb3, Kin28 and Ccl1 is assembled onto the core TFIIH through the Tfb3–Rad3 interaction, forming the 10 subunit holo TFIIH.