Abstract
An extreme thermophile Thermus thermophilus is a model organism for structural biology and systems biology, and the
so-called “Structural and Functional Whole-Cell Project for T. thermophilus HB8” is in progress. Its genomic sequence composed of a chromosome, a megaplasmid pTT27, and a plasmid pTT8 is available. We revealed that this model organism was a polyploid bacterium harboring four to five copies of the chromosome in a cell. The finding is not
surprising, as Deinococcus radiodurans, an extremely radioresistant bacterium closely related to Thermus, is well
known to be polyploid. Although the polyploidy of T. thermophilus might allow for genomic DNA protection, maintenance, and repair at elevated growth temperatures, it often complicates the recognition of an essential gene in
genetic analyses. We also demonstrated a third plasmid pVV8 in the HB8, and determined its sequence. Limited
information and an unfortunate dropout in the substrain, whose genomic sequence was determined, would have
prevented the plasmid from coming to public attention. The
intrinsic circular plasmid, which was estimated to be six to
ten copies in a cell, is 81,151 bp and its G+C content is 68%.
The pVV8 was suggested to be nonessential for cell growth.
