Developmental gene regulation in Giardia lamblia: first evidence for an encystation-specific promoter and differential 5 ' mRNA processing

Giardia lamblia must encyst to survive in the environment and subsequently infect new hosts. We investigated the expression of glucosamine-6-phosphate isomerase (Gln6PI), the first enzyme required for biosynthesis of N-acetyl galactosamine, for the major cyst wall polysaccharide. We isolated two Gln6 PI genes that encode proteins with large areas of identity, but distinctive central and terminal regions. Both recombinant enzymes have comparable kin etics. Interestingly, these genes have distinct patterns of expression. Gln 6PI-A has a conventional, short 5' untranslated region (UTR), and is expres sed at a low level during vegetative growth and encystation. The Gln6PI-B g ene has two transcripts - one is expressed constitutively and the second sp ecies is highly upregulated during encystation. The non-regulated Gln6PI-B transcript has the longest 5'-UTR known for Giardia and is 5' capped or blo cked, In contrast, the Gln6PI-B upregulated transcript has a short, non-cap ped 5'-UTR, A small promoter region (< 56 bp upstream from the start codon) is sufficient for the regulated expression of Gln6PI-B, Gln6PI-B also has an antisense overlapping transcript that is expressed constitutively. A sho rter antisense transcript is detected during encystation. This is the first report of a developmentally regulated promoter in Giardia, as well as evid ence for a potential role of 5' RNA processing and antisense RNA in differe ntial gene regulation.