Histone-fold superfamily

Histones mediate DNA organisation and plays a dominant role in regulating eukaryotic transcription. The histone-fold consists of a core of three helices, where the long middle helix is flanked at each end by shorter ones. Proteins displaying this structure include the nucleosome core histones, which form octomers composed of two copies of each of the four histones, H2A, H2B, H3 and H4; archaeal histone, which possesses only the core domain part of eukaryotic histone; and the TATA-box binding protein (TBP)-associated factors (TAF), where the histone fold is a common motif for mediating TAF-TAF interactions. TAF proteins include TAF(II)18 and TAF(II)28, which form a heterodimer, TAF(II)42 and TAF(II)62, which form a heterotetramer similar to (H3-H4)2, and the negative cofactor 2 (NC2) alpha and beta chains, which form a heterodimer. The TAF proteins are a component of transcription factor IID (TFIID), along with the TBP protein. TFIID forms part of the pre-initiation complex on core promoter elements required for RNA polymerase II-dependent transcription. The TAF subunits of TFIID mediate transcriptional activation of subsets of eukaryotic genes. The NC2 complex mediates the inhibition of TATA-dependent transcription through interactions with TBP.