RESEARCH ARTICLE Summary. The members of the Deinococcus-Thermus phylum, which include many species that are resistant to extreme radiation, as well as several thermophiles, have been recognized solely on the basis of their branching patterns in 16S rRNA and other phylogenetic trees. No biochemical or physiological characteristic is currently known that is unique to this group of species. To identify genes/proteins that are exclusive of this group of species, systematic protein basic local alignment tool (Blastp) searches were carried out on each open reading frame (ORF) in the genome of Deinococcus radiodurans. These stud- ies identified 65 proteins that were only found in all three sequenced Deinococcus-Thermus genomes (viz. D. radiodurans, D. geothermalis and Thermus thermophilus), but not in any other bacteria. In addition, these studies also identified 206 pro- teins that are exclusively found in the two Deinocococci species, and 399 proteins that are unique to D. radiodurans. The identified proteins, which represent a genetic repertoire distinctive to the Deinococcus-Thermus group, or to Deinococci species, provide novel molecular markers for their identification and characterization. The cellular functions of most of these proteins are not known and their studies should prove useful in identifying novel biochemical and physiological characteris- tics that are exclusive of these groups of bacteria and also those responsible for the extreme radiation resistance of Deinococci. [Int Microbiol 2007; 10(3):201-208] Key words: Deinococcus spp. · Thermus thermophilus · Deinococci-specific proteins · radiation-resistant bacteria · extremophilic bacteria · ORFans proteins · lateral gene transfer Introduction The Deinococcus-Thermus group of species has enormous biochemical, physiological and phenotypic diversity [12]. While most species from this phylum stain gram-negative, and all of them contain an outer membrane, which is a dis- tinctive feature of gram-negative bacteria [16], those belong- ing to the genus Deinococcus have a thick layer of peptido- glycan that causes them to stain gram-positive [12,28]. Of the two main groups that comprise this phylum, the Deinococci species are known for their very high resistance to ionizing radiations; they can withstand up to 150 kGy (1.5 Mrads), which is more than 100-fold increased resistance as com- pared to Escherichia coli [3,6,12,26,27]. In addition, these organisms are also more resistant to ultraviolet radiation, oxidizing agents, and dessicating conditions [6,12,33]. In contrast to the Deinococci, the Thermales are highly ther- mophilic organisms; they inhabit niches up to 80 o C, but they do not have unusual radiation nor stress resistance capabili- ties [12]. The Deinococcus-Thermus phylum comprised two INTERNATIONAL MICROBIOLOGY (2007) 10:201-208 DOI: 10.2436/20.1501.01.28 ISSN: 1139-6709 www.im.microbios.org *Corresponding author: R.S. Gupta Department of Biochemistry McMaster University Hamilton, Ontario, Canada L8N 3Z5 Tel. +1-9055259140 ext. 22639. Fax +1-9055229033 E-mail: gupta@mcmaster.ca This article contains supporting information [SI] ONLINE, at: www.im.microbios.org Emma Griffiths, Radhey S. Gupta* Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada Received 23 June 2007 · Accepted 30 July 2007 Identification of signature proteins that are distinctive of the Deinococcus-Thermus phylum