Abstract
The Giardia lamblia cyst wall is required for survival outside the host and infection. Three cyst wall protein (cwp) genes identified to date are highly up-regulated during encystation. However, little is known of the molecular mechanisms governing their gene regulation. Messenger RNAs containing premature stop codons are rapidly degraded by a nonsense-mediated mRNA decay (NMD) system to avoid production of non-functional proteins. In addition to RNA surveillance, NMD also regulates thousands of naturally occurring transcripts through a variety of mechanisms. It is interesting to know the NMD pathway in the primitive eukaryotes. Previously, we have found that the giardial homologue of a conserved NMD factor, UPF1, may be functionally conserved and involved in NMD and in preventing nonsense suppression. In this study, we tested the hypothesis that NMD factors can regulate some naturally occurring transcripts in G. lamblia. We found that overexpression of UPF1 resulted in a significant decrease of the levels of CWP1 and cyst formation and of the endogenous cwp1-3, and myb2 mRNA levels and stability. This indicates that NMD could contribute to the regulation of the cwp1-3 and myb2 transcripts, which are key to G. lamblia differentiation into cyst. Interestingly, we also found that UPF1 may be involved in regulation of eight other endogenous genes, including up-regulation of the translation elongation factor gene, whose product increases translation which is required for NMD. Our results indicate that NMD factor could contribute to the regulation of not only nonsense containing mRNAs, but also mRNAs of the key encystation-induced genes and other endogenous genes in the early-diverging eukaryote, G. lamblia.
Highlights
Giardia lamblia is a major cause of outbreaks of waterborne diarrheal disease worldwide, which contributes greatly to malnutrition and malabsorption in children [1,2,3]
In addition to RNA surveillance, nonsensemediated mRNA decay (NMD) factors function in regulating the abundance of some naturally occurring mRNAs [22,27]
We found that NMD could be present in G. lamblia because the results showed that the mRNA produced from the luciferase gene with a stop codon under the control of the cwp1 promoter was decayed compared with wild type luciferase mRNA [34]
Summary
Giardia lamblia is a major cause of outbreaks of waterborne diarrheal disease worldwide, which contributes greatly to malnutrition and malabsorption in children [1,2,3]. Like Entamoeba histolytica and other intestinal protozoan parasites, G. lamblia undergoes differentiation from a pathogenic trophozoite form into a resistant infectious cyst form [3,4]. A Myb family transcription factor (Myb2) is encystation-induced and is involved in coordinating upregulation of the cwp genes [8]. Two GARP family transcription factors may be involved in transcriptional regulation of many different genes including the encystation-induced cwp gene and constitutive ran gene [9]. An ARID family transcription factor can bind to specific AT-rich Inr sequences and function as an important transactivator in the regulation of the cwp gene [10]. Little is known about regulation of mRNA stability of the cwp genes during giardial growth and differentiation
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