The DNA sequences were determined at the boundaries of the integrated copy of the archaebacterial genetic element SSV1. A 44 bp sequence present as a single copy on the 15.5 kb circular SSV1 DNA flanked the integrated copy as a direct DNA sequence repeat, suggesting that SSV1 integration occurred by recombination between this 44 bp SSV1 sequence and an identical sequence on the bacterial chromosome. At the left attachment site, a region encompassing the 44 bp attachment core sequence and the 31 nucleotides upstream of it displayed all characteristics expected for an arginine tRNA gene. An analysis of published attachment site sequences of other systems revealed that tRNA genes also constitute the bacterial attachment site in the case of three temperate phages and two transmissible plasmids in eubacteria, indicating a widespread occurrence of tRNA genes as integration target sites. This finding may be important for the understanding of mechanisms and evolution of site-specific recombination.

Sulfolobus acidocaldarius, strain B12, which harbours a double‐stranded DNA species both as a plasmid and in a linear form, which is integrated at a specific site of the chromosome, produces virus‐like particles upon u.v. irradiation. These particles contain the same circular DNA and a number of coat proteins and are probably surrounded by a lipid membrane. They are lemon shaped, 100 x 60 nm in size and carry tail structures at one pole. The host cell recovers and remains lysogenic after virus production. Though a large fraction of liberated particles is found attached to structures derived from the cells, neither adsorption nor infection of a number of Sulfolobus isolates has so far been observed.

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The transcription of the genome of the UV-inducible Sulfolobus virus-like particle SSV1 was studied. Eight different transcripts could be distinguished by Northern analysis that were present in uninduced cells and the coordinately increased in amount after UV induction of SSV1. Using single-stranded DNA probes from different parts of the genome, the approximate map positions of these RNAs and the directions of transcription were determined. In two cases, terminator read-through resulted in the formation of more than one RNA species from a single 5' end and therefore the eight different RNAs corresponded to only five different transcriptional starts. Two RNAs sharing a common 5' end encode SSV1 structural proteins. The 5' end of these transcripts was determined by S1 nuclease analysis. About 20 nucleotides upstream of the transcriptional start of these RNAs, there is an AT-rich region resembling putative promoter sequences which have been found at a similar distance 5' to the genes encoding stable RNAs in Thermoproteus. In addition to the eight constitutive transcripts, a UV-inducible RNA of 0.3 kb was mapped on the SSV1 genome. In contrast to all other RNAs, it was not detectable in uninduced cells and it is expressed shortly before the amplification and packaging of the SSV1 genome commences.

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