RNA polymerase I:
Transcribes only the 5.8S, 18S, and 28S rRNAs. Transcription occurs in nucleolus.
Transcribes them as one large 45S pre-rRNA containing one copy of each plus spacers (noncoding regions) inbetween (Fig. 6.16)
Subsequent processing yields the individual rRNAs.
Promoter lies directly upstream of initiation site. Promoter is specifically recognized by transcription factors UBF (upstream binding factor) and SL1 (selective factor 1).
A subunit of SL1 is TBP (TATA-binding protein)! (Fig. 6.17)
Discovered because yeast mutants in TBP show no transcription from promoters for RNA polymerase I, II and even III
---- central role for TBP in all nuclear transcription
But TBP not involved in specific recognition of promoter (no TATA box in RNA polymerase III promoters), instead specific recognition through other SL1 subunits
RNA polymerase III:
Transcribes only tRNA, 5S rRNA and some small RNAs.
Promoter lies downstream of transcription start site within the transcribed region! (Fig. 6.18)
5S rRNA:
note TBP as important subunit of TFIIIB!!
tRNA transcription slightly different as no TFIIIA involved. Instead, specific promoter recognition by TFIIIC which in turn recruits TFIIIB and polymerase III.
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