Giardia lamblia

On the basis of 16S rRNA analysis (1) it was concluded that the intestinal parasite Giardia lamblia, which does neither contain mitochondria nor a Golgi apparatus, represents the lowest known lineage in the eucaryal domain. In order to gain a better insight into the phylogenetic relationship between the three evolutionary domains of life, we included this primitive eukaryote in the comparison of sequences of the largest (A) subunits of eucaryal RNA polymerases and corresponding components (,B',A and A'plus A" respectively) from Bacteria and Archaea (2). The amino acid sequence NADFDGD(E/Q)M(N/A) is conserved in all ,3',A (respectively A') subunits of bacterial, eucaryal and archaeal RNA polymerases known so far (3, 4, 5). Frequently, an oligonucleotide primer derived from this sequence, specifically hybridized to three G. lamblia chromosomal DNA fragments, whether digested with SacI, AvaI, BamHI, XbaI, HindIII, SalI, PstI or PvuII (Figure 1). Thus it is probable that G. lamblia like other eukaryotes contains three different nuclear RNA polymerases. A PstI-fragment identified in this way was cloned into pBluescriptTM II SK+ and sequenced in both directions. It was found to contain a complete open reading frame (ORF) encoding a putative protein of 529 amino acids (from 1439 bp to 3022 bp) and the larger part of a