Diplomonads, parabasalids, as represented by trichomonads, and microsporidia are three protist lineages lacking mitochondria that branch earlier than all other eukaryotes in small subunit rRNA and elongation factor phylogenies. The absence of mitochondria and plastids in these organisms suggested that they diverged before the origin of these organelles. However, recent discoveries of mitochondrial-like heat shock protein 70 and͞or chaperonin 60 (cpn60) genes in trichomonads and microsporidia imply that the ancestors of these two groups once harbored mitochondria or their endosymbiotic progenitors. In this report, we describe a mitochondrial-like cpn60 homolog from the diplomonad parasite Giardia lamblia. Northern and Western blots reveal that the expression of cpn60 is independent of cellular stress and, except during excystation, occurs throughout the G. lamblia life cycle. Phylogenetic analyses position the G. lamblia cpn60 in a clade that includes mitochondrial and hydrogenosomal cpn60 proteins. The most parsimonious interpretation of these data is that the cpn60 gene was transferred from the endosymbiotic ancestors of mitochondria to the nucleus early in eukaryotic evolution, before the divergence of the diplomonads and trichomonads from other extant eukaryotic lineages. A more complicated explanation requires that these genes originated from distinct ␣-proteobacterial endosymbioses that formed transiently within these protist lineages.

We used molecular techniques to assess the phylogenetlc afflnlty of cultured and uncultured m~croorganis~ns from Toolik Lake, an ol~gotrophic lake In a r c t~c Alaska USA The phylogenetlc positions of cloned cultures of bacteria were determined by sequence analysls of PCR ampl~fled ribosomal RNA genes The Toollk Lake bacterial isolates showed a high degree of slmllarlty, 0 94 to 0 99, to a wlde variety of phyla that are well represented in the ribosomal RNA database The occurrence of species normally associated with a terrestrial habltat (Arthrobacter globiforn~is and B~~r k h o l d e n a solanacearum) or a more nutrient-rich environment (Cytophaga aquatills and Zoogloea r a m~y e r a ) suggests a particle-associated origln for these cell types consistent with the fact that w e used an unflltered sample In contrast, the analysls of rRNA genes cloned from a complex natural DNA community indlcated the predominance of beta-proteobacteria closely related to the rRNA hamology group I1 pseudomonads Alcalrgenes eutrophus and Pseudomonaspickett~l However, 2 of therRNA g e n e clones are deeply branching relatives (s~rnllarity = 0 88) of the alpha-proteobacteria SARI 1 cluster, previously detected only In marine environments Thls finding lndlcates a widespread aquatic dlstnbutlon for thls recently descnbed group KEY WORDS: Bactel-]a 1 6 s rRNA. Arctic. SARll