VIRUSES, PLASMIDS AND OTHER GENETIC ELEMENTS OF THERMOPHILIC AND HYPERTHERMOPHILIC ARCHAEA

We review and update the work on genetic elements, e.g., viruses and p lasmids (exluding IS elements and transposons) in the kingdom Crenarch aeota (Thermoproteales and Sulfolobales) and the orders Thermococcales and Thermoplasmales in the kingdom Euryarchaeota of the archael domai n, including unpublished data from our laboratory. The viruses of Cren archaeota represent four novel virus families. The Fuselloviridae repr esented by SSV1 of S. shibatae and relatives in other Sulfolobus strai ns have the form of a tailed spindle. The envelope is highly hydrophob ic. The DNA is double-stranded and circular. Members of this group hav e also been found in Methanococcus and Haloarcula. The Lipothrixvirida e (e.g., TTV1 to 3) have the form of flexible filaments. They have a c ore containing linear double-stranded DNA and DNA-binding proteins whi ch is wrapped into a lipid membrane. The 'Bacilloviridae' (e.g., TTV4 and SIRV) are stiff rods lacking this membrane, but also featuring lin ear double-stranded DNA and DNA-binding proteins. Both virus types car ry on both ends structures involved in the attachment to receptors. Bo th types are represented in Thermoproteus and Sulfolobus. The droplet- formed novel Sulfolobus virus SNDV represents the 'Guttaviridae' conta ining circular double-stranded DNA, Though head and tail viruses dista ntly resembling T phages or lambdoid phages were seen electronmicrosco pically in solfataric water samples, no such virus has so far been iso lated. SSV1 is temperate, TTV1 causes lysis after induction, the other viruses found so far exist in carrier states. The hosts of all but TT V1 survive virus production. We discuss the implications of the nature of these viruses for understanding virus evolution. The plasmids foun d so far range in size from 4.5 kb to about 40 kb Most of them occur i n high copy number, probably due to the way of their detection. Most a re cryptic, pNOB8 is conjugative, the widespread pDL10 alleviates in a n unknown way autotrophic growth of its host Desulfurolobus by sulfur reduction. The plasmid pTIK4 appears to encode a killer function. pNOB 8 has been used as a vector for the transfer of the Inc S (beta-galact osidase) gene into a mutant of S. solfataricus.