Transcription in Archaea

Before the late 1970s, life was viewed as being dividedinto two distinct domains, the prokaryotes and the eukaryotes. The eukaryotes possessed nuclei, extensivesubcellular structures, and compartmentalization andcontained complex RNA polymerases. In contrast, theprokaryotes lacked these structures and had simpler RNApolymerases. However, in 1977, this dogma was challenged by the discovery, based on phylogenetic analysisof 16S and 18S rRNA sequences, that the prokaryoteswere composed of two distinct groups: the Bacteria andthe Archaea (Woese and Fox 1977). Subsequent studieshave led to a confirmation of the conclusion that the Archaea are a coherent domain of life as distinct from bacteria as they are from Eucarya. Divergence of the threelineages appears to have occurred very early in the evolution of life. Moreover, it has been proposed recentlythat bacteria diverged before the Eucarya and Archaeasplit (Fig. 1) (Doolittle and Brown 1994). Thus, Archaeaand Eucarya shared a period of common evolutionary history distinct from Bacteria, leading to the establishmentof biochemical pathways in the progenitors of Archaeaand Eucarya distinct from analogous processes in Bacteria. Perhaps one of the clearest examples of this lies in thestriking similarities between the archaeal and eucaryalbasal transcription machineries and their differences fromthose in the bacterial systems...