Abstract
Gram-positive bacteria of the genus Exiguobacterium have been repeatedly isolated from Siberian permafrost ranging in age from 20,000 to 2 to 3 million years and have been sporadically recovered from markedly diverse habitats, including microbial mats in Lake Fryxell (Antarctic), surface water, and food-processing environments. However, there is currently no information on genomic diversity of this microorganism or on the physiological strategies that have allowed its survival under prolonged freezing in the permafrost. Analysis of the genome sequence of the most ancient available Exiguobacterium isolate (Exiguobacterium sp. strain 255-15, from 2 to 3 million-year-old Siberian permafrost) revealed numerous putative transposase sequences, primarily of the IS200/IS605, IS30, and IS3 families, with four transposase families identified. Several of the transposase genes appeared to be part of insertion sequences. Southern blots with different transposase probes yielded high-resolution genomic fingerprints which differentiated the different permafrost isolates from each other and from the Exiguobacterium spp. type strains which have been derived from diverse surface habitats. Each of the Exiguobacterium sp. strain 255-15 transposases that were used as probes had highly conserved homologs in the genome of other Exiguobacterium strains, both from permafrost and from modern sites. These findings suggest that, prior to their entrapment in permafrost, Exiguobacterium isolates had acquired transposases and that conserved transposases are present in Exiguobacterium spp., which now can be isolated from various modern surface habitats.