IBP39 recognises the initiator which is solely responsible for transcription start site selection. IBP39 contains an N-terminal Inr binding domain connected to a C-terminal domain. The C domain structure indicates that it interacts with the T. vaginalis RNAP II large subunit C-terminal domain. Binding of IBP39 to Inr recruits RNAP II and initiates transcription [1].

1: Structural basis of core promoter recognition in a primitive eukaryote. Schumacher MA, Lau AO, Johnson PJ; Cell. 2003;115:413-424. PMID:14622596