ARCHITECTURE OF A YEAST U6 RNA GENE PROMOTER

The promoters of vertebrate and yeast U6 small nuclear RNA genes are s tructurally dissimilar, although both are recognized by RNA Polymerase III. Vertebrate U6 RNA genes have exclusively upstream promoters, whi le the U6 RNA gene from the yeast Saccharomyces cerevisiae (SNR6) has internal and downstream promoter elements that match the tRNA gene int ragenic A- and B-block elements, respectively. Substitution of the SNR 6 A or B block greatly diminished U6 RNA accumulation in vivo, and a s ubcellular extract competent for RNA polymerase III transcription gene rated nearly identical DNase I protection patterns over the SNR6 downs tream B block and a tRNA gene intragenic B block. We conclude that the SNR6 promoter is functionally similar to tRNA gene promoters, althoug h the effects of extragenic deletion mutations suggest that the downst ream location of the SNR6 B block imposes unique positional constraint s on its function. Both vertebrate and yeast U6 RNA genes have an upst ream TATA box element not normally found in tRNA genes. Substitution o f the SNR6 TATA box altered the site of transcription initiation in vi vo, while substitution of sequences further upstream had no effect on SNR6 transcription. We present a model for the SNR6 transcription comp lex that explains these results in terms of their effects on the bindi ng of transcription initiation factor TFIIIB.