In Trichomonas vaginalis, thought to be the earliest extant eukaryote, the sole initiator element for control of the start of transcription 
is Inr, and this is recognised by the initiator binding protein IBP39. IBP39 contains an N-terminal Inr binding domain (IBD, represented by this entry)
connected via a flexible, proteolytically sensitive, linker (residues 127-145) to a C-terminal domain. The IBD structure reveals a winged-helix-wing 
conformation with each element binding to DNA, the central helix-turn-helix contributing the majority of the specificity-determining contacts with
the Inr core motif TCAPy(T/A). The binding of IBP39 to the Inr directly recruits RNA polymerase II and in this way initiates transcription.