The eukaryotic ribosme contains four rRNAs (18S, 28S, 5.8S and 5S). Ribosomal DNA (rDNA) are sequences encoding ribosomal RNA. The nucleolus consists of expanded chromosomal loops that carry transcriptional units encoding 18S-5.8S-28S ribosomal RNA.The 5S rDNA is coded separately and trancribed out of nucleolus. Giardia lamblia is one of the most widerspread intestinal protozoan parasites without nucleolus. And previous study showed that there is no electrophoresis band of 5S rRNA was found in experiment. We investigated the characterization of the rDNA in Giardia lamblia. Our results are as follows:
1. The basic repeat unit of 5.4 kp is smaller than a lot of eukaryotes. And the 18S and 28S rRNA are smaller than that of most eukaryotes, even of some baceria as well. The 18S-5.8S-28S rRNA gene are extraordinarily G:C rich. It is noteworthy that the overall G:C conetent of the G. lamblia genome is not similarly skewed. Most of the rRNA genes in Giardia lamblia genome is not complete. The arrangement of Giardia rDNA is not regular as typical eukaryotes too. There is no region as typical eukaryotes has tandemly repeated genes form arrays in one or several chromes.
2. We tried to identify the 5S rRNA gene and the related proteins (transcription IIIA and ribosomal protein L5) in Giardia lamblia genome, but no transcription factor IIIA homologous gene was detected. RT-PCR experiment proved that the Giardia 5S rRNA gene and L5 gene are transcribed actively. The proposed Giardia lamblia mature 5S rRNA was folded in accordance with the typical 5S rRNA secondary structure and contained most of the conserved sites in the structure. Primary sequence comparison revealed a relatively high conservation of Giardia L5 when compared with other organisms, and there is 5S rRNA binding domain on Giardia L5 protein, suggesting that the Giardia 5S rRNA may be incorporated into ribosome with L5 ribosomal protein as typical eukaryotes. The absence of transcription factor IIIA suggested that the initiation of Giardia 5S rRNA gene transcription is very different from those in typical eukaryotes. Previous study showed that four of five RNA polymerase III specific subunits were absent in Giardia genome. However, the Giardia RNA polymerase III apparatus could transcribe the 5S rRNA gene，which implied that these RNA polymerase specific subunits may be evolved for other function.
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