Submitted to: American Society for Microbiology
Publication Type: Review Article
Publication Acceptance Date: April 24, 2009
Publication Date: N/A
Interpretive Summary: Giardiasis is a diarrheal disease of humans and animals caused by the protozoan parasite, Giardia lamblia. The disease, which affects over 200 million persons each year, is generally transmitted between humans, and between humans and animals (zoonotic cycle), through ingestion of water that is contaminated with the cyst stage of the parasite. In this paper, a review of molecular aspects of G. lamblia is presented, with an emphasis on evolutionary history of the parasite, and potential vaccine targets to prevent or treat infection. The complete DNA sequence of the G. lamblia genome is now available, and has allowed for rapid discovery of genes coding for surface and internal proteins, enzymes involved in energy generation, and DNA sequences, such as those coding for ribosomal RNA (rRNA), that are conserved between bacteria and eukaryotes. Comparison of protein and rRNA gene sequences between G. lamblia, prokaryotes, and eukaryotes has confirmed the view that Giardia diverged early in eukaryotic evolution. Study of G. lamblia DNA sequences has provided insight on evolutionary history of eukaryotic features such as mitochondria and a cytoskeleton. Yet, Giardia appears to have many properties in common with bacteria, such as a highly compact genome, and a simplified gene structure. Most researchers agree that Giardia lost many eukaryotic traits in adapting to growth in an anaerobic environment. Molecular techniques in conjunction with biological studies have provided evidence for different assemblages and possibly species of Giardia. The rapid advance in many diverse areas of G. lamblia biology and molecular biology is a testament to the value of whole genome sequencing efforts.
Technical Abstract: The rapid advance in our understanding of the biology of Giardia lamblia over the last several years is due in part to the complete DNA sequencing of the 11.7 Mb genome of this diplomonad. Insight on the molecular nature of G. lamblia has been gained by searching the genome using query sequences from prokaryotic and eukaryotic organisms. Features such as transcriptional regulation, genome organization, the nature of upstream (e.g. promoter, initiator) and downstream (e.g. polyadenylation signal) sequences, encystation and exycstation processes, vsp gene expression, the role of protein kinases in regulating gene expression, and metabolic pathways unique to this anaerobic protozoan continue to be uncovered. The two-fold nature of G. lamblia, that is retaining features of prokaryotes (simplied gene organization, anaerobic metabolism) and eukaryotes (nucleus, endomembrane system, cytoskeleton), continues to be borne out by studies of its genome. Thus, the term “a bacterium in a eukaryotic cloak” put forth by Upcroft and Upcroft (162) over a decade ago remains appropriate.