The human polymerase I promoter contains the necessary sequences to
recruit human RNA polymerase I (transcribes rRNA genes) and
initiate transcription, rather than RNA polymerase II (transcribes
protein coding genes) or RNA polymerase III (generally transcribes
tRNA genes). In human cells, RNA Pol I only recognizes one promoter
to synthesize 45S pre-rRNA, which will be processed into 5.8S, 18S
and 28S rRNA. RNA Pol I synthesizes a triphosphate group at the 5'
end of the transcripts.