Characterization of upstream activating sequences involved in activation and regulation of pho4 expression in Schizosaccharomyces pombe.

In this study, a short region of the pho4 promoter located just upstream of the TATA box, called the upstream activating sequence (UAS), was shown to be responsible for both activation and regulation of pho4 expression in Schizosaccharomyces pombe. This cis-acting sequence is able to activate transcription when placed upstream of the minimal promoter of the constitutively expressed adh gene, and to ensure regulation by thiamine. The organisation of the pho4 promoter is remarkable in that UAS and TATA box are very closely associated. This proximity appears to be essential for efficient transcriptional regulation. Indeed, only slight variations in transcript levels were observed under derepression conditions when the UAS was moved away from the TATA box, while regulation of transcript levels appeared to be strongly affected. This observation suggests the existence of a negative regulatory element whose action requires very close association of UAS and TATA box. Surprisingly, when UAS was moved about 40 bp away from the adh TATA box, residual but very significant repression of gene expression by thiamine, which apparently does not result from transcriptional regulation, was revealed by measuring protein production. These data prompted us to hypothesize the existence of two distinct and cooperative mechanisms of regulation involving specific factor binding to the UAS, the first one controlling transcript levels and the second acting posttranscriptionally.