The Genus Thermus and Relatives
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The Genus Thermus and Relatives MILTON S. DA COSTA, FRED A. RAINEY AND M. FERNANDA NOBRE
Introduction In 1969, Brock and Freeze described a thermophilic organism that they named Thermus aquaticus. Since then the bacteria of the genus Thermus have become the archetypal thermophilic bacteria even though other organisms have been described that grow at much higher temperatures. Not only are these organisms easy to grow, but some strains are transformable and are rapidly becoming the mainstays of molecular biology of thermophilic bacteria. The importance of these organisms to our knowledge of thermophilic lifestyles was recently reaffirmed by the complete genome sequencing of Thermus thermophilus HB27 (Henne et al., 2004). This chapter should probably be entitled the “Family Thermaceae” or the “Order Thermales” as defined by us (da Costa and Rainey, 2001a; Rainey and da Costa, 2001), but we have decided to retain the title of the original chapter in The Prokaryotes as an indication of continuity and because the species of the genus Thermus continue to be the most studied organisms of this family. Moreover, the four new genera of the family Thermaceae have only recently been proposed to accommodate thermophilic or slightly thermophilic bacteria that are distinct but closely related to the species of the genus Thermus. These recently proposed genera are Meiothermus (Nobre et al., 1996b), Marinithermus (Sako et al., 2003), Oceanithermus (Miroshnichenko et al., 2003a; Mori et al., 2004) and Vulcanithermus (Miroshnichenko et al., 2003b). The organisms of these genera constitute the family Thermaceae, which together with the species of the genus Deinococcus constitute a deep branching lineage designated the “Phylum Deinococcus/Thermus” (Rainey et al. 1997; Battista and Rainey, 2001).
The Genera Thermus and Meiothermus Taxonomy and Phylogeny The genus Thermus currently comprises eight validly described species, namely T. aquaticus
(Brock and Freeze, 1969), T. thermophilus (T. Oshima and Imahori, 1974; Manaia et al., 1994), T. filiformis (Hudson et al., 1987b), T. brockianus (Williams et al., 1995), T. oshimai (Williams et al., 1996), T. scotoductus (Kristjánsson et al., 1994), T. antranikianii and T. igniterrae (Chung et al., 2000). Comparison of the 16S rDNA sequences of the type strains of each of the eight validly described species of the genus Thermus shows the 16S rDNA sequence similarities to be in the range 91.2–96.4% (Fig. 1). Thermus oshimai is the most unrelated of the species of the genus Thermus as reflected by the 16S rDNA sequence similarity values. The other species of the genus Thermus have 16S rDNA sequence similarities in the range 94–96%. Within each species the 16S rDNA similarity values are in the range of 98.9– 99.7% for T. aquaticus, 99.9–100% for T. brockianus, 99.2–99.9% for T. filiformis, 99.8–100% for T. oshimai, 98.7–99.9% for T. scotoductus, 99.4– 100% for T. thermophilus, and 99.9–100% for T. igniterrae and T. antranikianii (Chung et al., 2000). These valu
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