LESS IS MORE: THE IMPLICATIONS OF GENOMIC MINIMALISM ON THE REGULATION OF GENE EXPRESSION IN THE PARASITE GIARDIA LAMBLIA

Abstract

<italic>Giardia lamblia<italic> is one of the most common eukaryotic intestinal parasites in the world. A theme of simplified cellular machinery characterizes much of its biology with interesting consequences: a greatly reduced set of transcription factors result in an abundance of antisense mRNA transcripts with no known functions. This reduced level of transcriptional control suggests that gene expression is largely regulated either pre- or post-transcriptionally. `Pre-transcriptional' implies changes to chromatin structure and includes DNA and histone methylation and histone acetylation. `Post-transcriptional' implies the regulation of mRNA stability and/or translation. The two major components of the research presented here have focused in turn on each of these stages of regulation.

Pre-transcriptional regulation has received relatively little attention in Giardia, but it is known that histone acetylation plays a role in the expression of antigenic surface proteins and is regulated during development. Therefore we sought to determine if DNA methylation was present in Giardia. We successfully documented the presence of DNA methylation and further showed that despite its remarkably low abundance, DNA methylation is required for parasite growth and development. We are currently seeking an explanation for this dependence on DNA methylation and suspect a role in maintenance of genome stability.

Likewise, although a small number of studies have looked at specific post-transcriptional regulatory mechanisms, none had previously performed a comprehensive analysis of post-transcriptional regulation in Giardia. We undertook this goal by first performing a comprehensive identification and analysis of Giardia's RNA decay machinery using an in silico approach and secondly by analyzing the stability of mRNA sense and antisense transcripts to identify the parasite's ability to differentially handle its diverse RNA population. We discovered that Giardia possesses many of the RNA decay complexes typically found in eukaryotic organisms, but appears to be missing components that play roles in noncoding transcript decay. These in silico findings were supported by our mRNA decay analyses that demonstrated that cognate pairs of sense and antisense transcripts possess roughly equivalent decay rates, suggesting that Giardia cannot differentiate between coding and noncoding transcripts. We are interested in further understanding the molecular consequences of this unusual transcript stability.