Archaebacteria Vs Metabacteria
Over 550 complete nucleotide sequences of 5S ribosomal RNAs from cytoplasm, chloroplasts, and mitochondria are now available. A phylogenetic tree deduced from them suggests that all extant organisms are classified into three major lineages: eubacteria, metabacteria (archaebacteria), and eukaryotes. In the eubacterial branch, several groups, such as Gram-negative bacteria, cyanobacte ria, Gram-positive bacteria, and actinobacteria, diverged during the early stage of evolution. Metabacteria and eukaryotes separated after the emergence and divergence of eubacteria.
By using 16S rRNA sequences, many trees are also now available [1–4]. However, there are some instances where 5S and 16S rRNA sequences give discrepancies larger than those expected from statistical error . The explanation is that the discrepancy, especially that of metabacteria versus archaebacteria, is due to a drastic change of substitution rates in 16S rRNAs.
Although metabacteria form a very unique group in bacteria, the 5S rRNA tree supports the view that these bacteria still belong to the prokaryotic domain together with eubacteria. Therefore, life on this planet is seen as comprising two domains, the eukaryotic domain and the prokaryotic domain (the latter includes the metabacterial and eubacterial kingdoms), but not comprising the three domains of Archaea, Bacteria, and Eukarya by Woese et al. .
KeywordsSubstitution Rate Unweighted Pair Group Average Method Prokaryotic Domain Primary Kingdom Thermoacidophilic Archaebacterium
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