20.5RNA polymerase Ill uses both downstream and upstream promoters Preinitiation complex in eukaryotic transcription describes the assembly of transcription factors at the promoter before rna polymerase binds 消当Preinitiation complex in eukaryotic transcription describes the assembly of transcription factors at the promoter before RNA polymerase binds. 20.5 RNA polymerase III uses both downstream and upstream promoters