Comparative studies on structure and function of archaebacterial elongation factors indicate the phylogenetic diversity of the urkingdom

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The sequences of ADP-ribosylated tryptic peptides from three archaebacterial elongation factors — aEF-2 from Methanobacterium thermoautotrophicum, from Sulfolobus acidocaldarius and from Desulfurococcus mucosus — were established. The rates of ADP-ribosylation by diphtheria toxin of these factors and of two others — aEF-2 from Methanococcus vannielii and from Thermoplasma acidophilum — were determined. Enzymatically active EF's and purified ribosomes from the two sulphur metabolizing strains were prepared. The phenylalanine polymerization activity of factors and ribosomes from these two strains and from T. acidophilum and Mc. vannielii in various combinations was analyzed. The results of these experiments allow, in connection with previously published findings (Klink et al., 1983; Gehrmann et al., 1985), the following statements and conclusions: Comparison of the ADP-ribosylatable domains of aEF-2 from six of seven archaebacterial orders and of the four known sequences of eEF-2 reveals greater differences within the archaebacterial sequences than within eukaryotic ones or between particular archaebacterial and eukaryotic sequences. Only the two sulphur metabolizing strains have identical peptides. The universal consensus sequence comprises only four of nine amino acids, although non-conservative exchanges occur only in two positions. No correlation has been found between sequence differences and ADP-ribosylation rates. Concerning the cooperation of both factor types - EF-1 and EF-2 - with heterologous ribosomes, restrictions are found to exist within the archaebacterial kingdom which have never been observed within the other kingdoms, with the exception of mitochondrial ribosomes. These restrictions confirm the deep hiatus between the sulphur metbolizing orders and the other archaebacteria, and also clearly separate the Sulfolobales from the Thermoproteales. Thermoplasma seems to be more closely related to the methanogenic/halophilic branch. In summary, the results reported here may indicate that the archaebacterial kingdom split up into several lines at a very early date, the two sulphur metabolizing orders being the most ancient branches.