PHYLOGENETIC ANALYSIS OF 70-KD HEAT-SHOCK PROTEIN SEQUENCES SUGGESTS A CHIMERIC ORIGIN FOR THE EUKARYOTIC CELL-NUCLEUS

Background: The evolutionary relationships between archaebacteria, eub acteria and eukaryotic cells are of central importance in biology. The current view is that each of these three groups of organisms constitu tes a monophyletic domain, and that eukaryotic cells have evolved from an archaebacterial ancestor. Recent studies on a number of highly con served protein sequences do not, however, support this view and raise important questions concerning the evolutionary relationships between all extant organisms, particularly regarding the origin of eukaryotic cells. Results: We have used sequences of 70 kD heat shock protein (hs p70) - the most conserved protein found to date in all species - to ex amine the evolutionary relationship between various species. We have o btained two new archaebacterial hsp70 sequences from the species, Ther moplasma acidophilum and Halobacterium cutirubrum. A global comparison of hsp70 sequences, including our two new sequences, shows that all k nown archaebacterial homologs share a number of sequence signatures wi th the Gram-positive group of bacteria that are not found in any other prokaryotic or eukaryotic species. In contrast, the eukaryotic homolo gs are shown to share a number of unique sequence features with the Gr am-negative bacteria that are not present in any archaebacteria. Detai led phylogenetic analyses of hsp70 sequences strongly support a specif ic evolutionary relationship between archaebacteria and Gram-positive bacteria on the one hand, and Gramnegative bacteria and eukaryotes on the other. The phylogenetic analyses also indicate a polyphyletic bran ching of archaebacteria within the Gram-positive species. The possibil ity that the observed relationships are due to horizontal gene transfe rs can be excluded on the basis of sequence characteristics of differe nt groups of homologs. Conclusions: Our results do not support the vie w that archaebacteria constitute a monophyletic domain, but instead su ggest a close evolutionary linkage between archaebacteria and Gram-pos itive bacteria. Furthermore, in contrast to the presently accepted vie w, eukaryotic hsp70s show a close and specific relationship to those f rom Gram-negative species. To explain the phylogenies based on differe nt gene sequences, a chimeric model for the origin of the eukaryotic c ell nucleus involving fusion between an archaebacterium and a Gram-neg ative eubacterium is proposed. Several predictions from the chimeric m odel are discussed.