Transcription units for RNA polymerase I have a core promoter separated by about 70 base pairs from the upstream control element (UCE). UBF1 binds to both regions, after which SL1 can bind. RNA polymerase I then binds to the core promoter. The nature of the interaction between the factors bound at the upstream control element and those at the core promoter is not known. The sequence of the GC-rich region in the core promoter is shown.