Initiator and upstream elements in the alpha 2-tubulin promoter of Giardialamblia

Giardia lamblia, one of the earliest diverging eukaryotes and a major cause of diarrhea world-wide, has unusually short intergenic regions, raising qu estions concerning its regulation of gene expression. We have approached th is issue through examination of the alpha2-tubulin promoter and in particul ar investigated the function of an AT-rich element surrounding the transcri ption start site. Its placement and the ability of this sequence to direct transcription initiation in the absence of any other promoter elements is s imilar to the initiator element in higher eukaryotes. However, the sequence diversity of extremely short (8-10 bp) initiator elements is surprising, a s is their ability to independently direct substantial levels of transcript ion. We also identified a large AT-rich element located between - 64 and - 29 bp upstream of the transcriptional start site and show using both deleti ons and site-specific mutations of this region that sequences between - 60 and the start of transcription are important for promoter strength; interes tingly this AT-rich sequence is not highly conserved among different Giardi a promoters. These data suggest that while the overall structure of the cor e promoter has been conserved throughout eukaryotic evolution, significant variation and flexibility is allowed in element consensus sequences and rol es in transcription. In particular, the short and diverse sequences that fu nction in transcription initiation in Giardia suggest the potential for rel axed transcriptional regulation. (C) 2001 Elsevier Science B.V. All rights reserved.