second cyclin box found in archaeal transcription initiation factor IIB (TFIIB) and similar proteins
Archaeal TFIIB stabilizes TATA-binding protein (TBP) binding to an archaeal box-A promoter. It is also responsible for recruiting RNA polymerase II to the pre-initiation complex (DNA-TBP-TFIIB). TFIIB contains two cyclin boxes. This model corresponds to the second one. The cyclin box fold is generally a protein binding domain, but binds DNA in TFIIB.