9 Reverse Gyrase and Other Archaebacterial Topoisomerases

Abstract

There is controversy over whether or not prokaryotes should be divided into two separate kingdoms: eubacteria and archaebacteria. Archaebacteria have several distinct features, but most of these might be a consequence of their growth in extreme habitats, such as at high temperature for acidothermophiles, in high salt for extreme halophiles, and in an obligate anaerobic environment for methanogens. It is a difficult task to put these three archaebacteria into a single group, but they share common properties, such as sequence similarity in rRNA, high complexity of RNA polymerase subunits, and cell-wall structures devoid of peptideglycans. Figure 1 shows the proposed phylogenetic relationship, based on rRNA sequences, of archaebacteria to eubacteria or eukaryotes (Woese 1987).

It has often been suggested that in some aspects archaebacteria are more closely related to eukaryotes than prokaryotes (e.g., Zillig et al. 1985), although nuclear structures have never been shown in archaebacterial cells. An important issue in the study of archaebacteria is how chromosome structure is maintained under such extreme conditions, such as 75°C for acidothermophiles or 3–4 M KCl for halophiles. Under these conditions many proteins from other kingdoms dissociate from DNA, and the DNA itself would be expected not to have the canonical B-form configuration. To keep chromosomal structure intact, histone-like proteins or other DNA-binding proteins might play important roles (Notbohn 1982; Kimura et al. 1984; Kikuchi et al. 1986). Accordingly, specific topoisomerases might be needed to change local constraints on DNA and to construct tight DNA/protein complexes in obnoxious conditions. One of...