Pol III initiation complexes open the promoter spontaneously. Indeed, this is the general case for DNA-dependent RNA polymerases. Only pol II, with its requirement for TFIIH-directed and ATP-dependent promoter opening is exceptional. TFIIH introduces a layer of mechanism that is not in the repertoire of any other transcriptase. Thus, it is pol III-mediated transcription that is, from a mechanistic perspective, most directly comparable with archaeal and also bacterial transcription.
As promoter opening has been analyzed only in the S. cerevisiae this event is Inferred from the homologous pathway in yeast.