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Two conjugative plasmids (CPs) were isolated and characterized from the same 'Sulfolobus islandicus' strain, SOG2/4. The plasmids were separated from each other and transferred into Sulfolobus solfataricus. One has a high copy number and is not stable (pSOG1) whereas the other has a low copy number and is stably maintained (pSOG2). Plasmid pSOG2 is the first Sulfolobus CP found to have these characteristics. The genomes of both pSOG plasmids have been sequenced and were compared to each other and the available Sulfolobus CPs. Interestingly, apart from a very well-conserved core, 70% of the pSOG1 and pSOG2 genomes is largely different and composed of a mixture of genes that often resemble counterparts in previously described Sulfolobus CPs. However, about 20% of the predicted genes do not have known homologues, not even in other CPs. Unlike pSOG1, pSOG2 does not contain a gene for the highly conserved PlrA protein nor for obvious homologues of partitioning proteins. Unlike pNOB8 and pKEF9, both pSOG plasmids lack the so-called clustered regularly interspaced short palindrome repeats (CRISPRs). The sites of recombination between the two genomes can be explained by the presence of recombination motifs previously identified in other Sulfolobus CPs. Like other Sulfolobus CPs, the pSOG plasmids possess a gene encoding an integrase of the tyrosine recombinase family. This integrase probably mediates plasmid site-specific integration into the host chromosome at the highly conserved tRN[A.sup.Glu] loci.

Research has been conducted on the small plasmids pING2 and pING3 that could be conjugationally transferred with the help of complete conjugative plasmids (CPs). The comparison of their genomes with the genomes of the complete CPs has been carried out to distinguish between motifs and open reading frames involved in transferring them.

Research has been conducted on the proteinaceous toxins termed sulfolobicins. Results indicate that these toxins kill other strain cells of the same species.

Directed open reading frame (ORF) disruption and a serial selection technique in Escherichia coli and the extremely thermophilic archaeon Sulfolobus solfataricus allowed the identification of otherwise cryptic crucial and noncrucial vital open reading frames in the genome of the archaeal virus SSV1. It showed that the 15.5-kbp viral genome can incorporate a 2.96-kbp insertion without loss of viral function and package this DNA properly into infectious virus particles. The selection technique, based on the preferential binding of ethidium bromide to relaxed DNA and the resulting inhibition of endonuclease cleavage to generate a pool of mostly singly cut molecules, should be generally applicable. A fully functional viral shuttle vector for S. solfataricus and E. coli was made. This vector spreads efficiently through infected cultures of S. solfataricus, its replication is induced by UV irradiation, it forms infectious virus particles, and it is stable at high copy number in both S. solfataricus and E. coli. The classification of otherwise unidentifiable ORFs in SSV1 facilitates genetic analysis of this virus, and the shuttle vector should be useful for the development of genetic systems for Crenarchaeota.

Members of two species of a novel genus and family of thermoacidophilic archaea are isolated and characterized by means of DNA sequence analysis, molecular cloning, lip analysis and nucleotide analysis. The species, namely, Picrophilus oshimae and Picrophilus torridus, exhibit optimal growth at 60 degrees Celsius at maximum pH level of 0.7. Their morphology and physiological properties indicate that they are members of a novel family within the order of Thermoplasmales.

pNOB8 is a 45 kb archaeal plasmid in heterotrophic Sulfolobus that transfers horizontally, the first known with this ability, and its transfer mechanism, though still unknown, does not involve a virus. The plasmid copies at an extremely high rate, resulting in retarded cell growth. Six of the nine separate Sulfolobus strains showed the presence of the plasmid, making it potentially valuable for determining the Sulfolobus genetic system.