Protein synthesis in Giardia lamblia may involve interaction between a downstream box (DB) in mRNA and an anti-DB in the 16S-like ribosomal RNA.
Giardia lamblia, a parasitic protozoan, has been regarded as one of the most conserved eukaryotes evolved from the prokaryotes. One of its unique features appears to be the unusually short 5'-untranslated regions (UTR) (1-6 nucleotides (nts)) and the apparent absence of 5'-cap structures from its mRNAs. Transfection of the Giardia trophozoites with luciferase-encoding chimeric transcripts, flanked by the 5'- and 3'-ends of giardiavirus (GLV) (+)-strand RNA, indicated that the translational efficiency was enhanced by 5000-fold when the 5'-viral sequence extended 264 nts into the capsid coding region and fused with the luciferase open reading frame (ORF). A 13-nt downstream box (DB) was identified within this region which complements a 15-nt sequence between nts # 1382 and 1396 near the 3'-end of the Giardia 16S-like ribosomal RNA (the anti-DB). Deletion or scrambling of this DB in the mRNA leads to a significant loss of the translational efficiency in Giardia. A Shine-Dalgarno (SD)-like element was also identified at 9-14 nts upstream from the initiation codon in the viral (+)-strand RNA, but alteration of its sequence led to no change in translation. Using the sequence complementary to ribosomal anti-DB to probe the Giardia mRNAs available in the databases, each mRNA was found to contain a putative DB with an average length from 8 to 13 nts. It is thus possible that initiation of translation in Giardia may involve a DB in the coding region of mRNA that may bind to a putative anti-DB in the small ribosomal RNA through base pairing. This mechanism of ribosome recruitment, which finds a potential parallel in Escherichia coli, could illustrate a relatively close distance between Giardia and prokaryotes in terms of translation initiation, and may provide a model for studying the evolution of translation machinery.
Yu
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Department of Pharmaceutical Chemistry, University of California, San Francisco 94143-0446, USA.
Wang
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Botka
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Wang
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CC
D020121:5' Untranslated Regions; D000818:Animals; D020029:Base Pairing; D016829:Giardia lamblia / Q000235:genetics / Q000378:metabolism*; D019198:Giardiavirus / Q000235:genetics; D008156:Luciferases / Q000096:biosynthesis / Q000235:genetics; D014176:Protein Biosynthesis; D015800:Protozoan Proteins / Q000096:biosynthesis*; D012315:RNA Caps; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism*; D016053:RNA, Protozoan / Q000235:genetics / Q000378:metabolism*; D012336:RNA, Ribosomal, 16S / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D014162:Transfection
D020121:5' Untranslated Regions; D015800:Protozoan Proteins; D012315:RNA Caps; D012333:RNA, Messenger; D016053:RNA, Protozoan; D012336:RNA, Ribosomal, 16S; D011993:Recombinant Fusion Proteins; D008156:Luciferases