DEVELOPMENTALLY-REGULATED TRANSCRIPTS AND EVIDENCE OF DIFFERENTIAL MESSENGER-RNA PROCESSING IN GIARDIA-LAMBLIA

Although encystation and excystation are crucial to transmission of Gi ardia lamblia, little is known about the regulation of these very dist inct differentiation processes. Fingerprinting of giardial mRNA popula tions throughout the time course of differentiation demonstrated compl ex patterns in mRNA differential display. Certain transcripts appeared or increased, while others decreased or disappeared at specific times , in response to physiologic stimuli that mimic key stages in parasite descent through the host gastrointestinal tract. This approach has al lowed the direct identification of critical stages in differentiation, as well as isolation of genes which may be crucial to the development of G. lamblia. One stage-specific single copy gene (ENC6) whose trans cript is greatly upregulated during encystation was analyzed further. Partial sequence analysis revealed no correspondence with known genes. 3'-rapid amplification of cDNA ends (3'-RACE) analysis of ENC6 transc ripts at various times of encystation revealed two polyadenylation sit es. The more proximal site, 10 nucleotides past the single classic AGT AAA sequence, was utilized only during encystation and its transcript increased similar to 16-fold during the first 24 h of encystation. In contrast, a slightly divergent polyadenylation site 288 nucleotides do wnstream from the open reading frame (ORF) was used during both vegeta tive growth and encystation, although its transcript was present at lo w levels. These studies are the first evidence of differential mRNA pr ocessing in G. lamblia and suggest a potential role of the 3'-untransl ated region (3'-UTR) in modulating gene expression during differentiat ion of this primitive eukaryote.