Comparative genomics of Thermus thermophilus: Plasticity of the megaplasmid and its contribution to a thermophilic lifestyle.

The bacterium Thermus thermophilus grows at temperatures up to 85 degrees C and is equipped with thermostable enzymes of biotechnological interest. The recently decoded genomes of two strains of T. thermophilus, HB27 and HB8, each composed of a chromosome and a megaplasmid, must certainly encode specific strategies to encounter the thermophile challenge. Here, a genome comparison was undertaken to distinguish common functions from the flexible gene pool, which gave some clues about the biological traits involved in a thermophile lifestyle. The chromosomes were highly conserved, with about 100 strain-specific genes probably reflecting adaptations to the corresponding biological niche, such as metabolic specialities and distinct cell surface determinates including type IV pili. The two megaplasmids showed an elevated plasticity. Upon comparison and re-examination of their gene content, both megaplasmids seem to be implicated in assisting thermophilic growth: a large portion of their genes are apparently involved in DNA repair functions. About 30 plasmid-encoded genes exhibit sequence and domain composition similarity to a predicted DNA repair system specific for thermophilic Archaea and bacteria. Moreover, the plasmid-encoded carotenoid biosynthesis gene cluster is interlocked with genes involved in UV-induced DNA damage repair. This illustrates the importance of DNA protection and repair at elevated growth temperatures.