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.