Roberts S, Colbert T, Hahn S
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA.
Genes Dev. 1995 Apr 1;9(7):832-42. doi: 10.1101/gad.9.7.832.
The gene encoding yeast U6 snRNA that is transcribed by RNA polymerase III (Pol III) contains both a TATA box upstream of the transcription start site and a downstream binding site for the factor TFIIIC. This juxtaposition of elements typical of both Pol II- and Pol III-transcribed genes raises the question of how polymerase specificity is determined. The upstream U6 promoter containing the TATA box and transcription start site was shown previously to be transcribed by Pol III in vitro. We therefore tested whether the upstream promoter of yeast U6 encodes Pol III specificity. One model is that polymerase specificity is conferred by the homologous Pol II and Pol III transcription factors TFIIB and BRF1. However, we found no specificity in the binding of BRF1 or TFIIB to TATA-containing promoters of genes specifically transcribed by Pol III or Pol II. Yeast strains deficient for Pol II or Pol III transcription were employed to examine U6 polymerase specificity in vivo. We find that the U6 upstream promoter is Pol II-specific in vivo and is converted to Pol III specificity by TFIIIC. Thus, preferential recruitment of TFIIIB by TFIIIC probably excludes the Pol II general factors and promotes Pol III transcription, thereby determining polymerase specificity.
由RNA聚合酶III（Pol III）转录的酵母U6小核RNA（snRNA）编码基因，在转录起始位点上游既含有一个TATA框，又含有转录因子TFIIIC的下游结合位点。这种同时具有Pol II和Pol III转录基因典型元件的并列情况，引发了聚合酶特异性是如何确定的问题。先前已表明，含有TATA框和转录起始位点的上游U6启动子在体外可由Pol III转录。因此，我们测试了酵母U6的上游启动子是否编码Pol III特异性。一种模型认为，聚合酶特异性是由同源的Pol II和Pol III转录因子TFIIB和BRF1赋予的。然而，我们发现BRF1或TFIIB与Pol III或Pol II特异性转录基因的含TATA框启动子的结合并无特异性。利用缺乏Pol II或Pol III转录的酵母菌株在体内检测U6聚合酶的特异性。我们发现，U6上游启动子在体内是Pol II特异性的，并且通过TFIIIC转变为Pol III特异性。因此，TFIIIC对TFIIIB的优先招募可能排除了Pol II通用因子，并促进了Pol III转录，从而决定了聚合酶特异性。
