Summary: Exchange and recombination of chromosomal markers is an intrinsic genetic property of the thermoacidophilic archaeon that has not been thoroughly characterized. To clarify the mechanism and experimental usefulness of this process, the frequency of prototrophs produced from mixtures of two pyrimidine auxotrophs under a variety of conditions was determined. The apparent efficiency of genetic exchange was essentially independent of the density of cells deposited on the surface of solid media. Furthermore, recombinant formation could initiate in liquid suspensions, as indicated by high recombinant frequencies resulting from mixtures plated at low cell densities, and the formation of recombinants at equal or higher frequencies in liquid suspensions that were never plated. Apparent initiation of genetic exchange in liquid at 22 °C was not prevented by DNase, prior digestion of parental cells with protease from , or any other non-lethal chemical agent tested. The results support prior indications that chromosomal marker exchange in proceeds via conjugation, and further indicate that this conjugation can initiate quickly in dilute liquid suspension. The mating system of thus appears physiologically distinct from that of but perhaps similar to conjugational transfer of plasmid pNOB8. The frequency of recombinants formed in these assays (10-10 per c.f.u.) greatly exceeds the number of spontaneous forward mutational events per generation for biosynthetic genes in This suggests that chromosomal exchange has the potential to influence the genetic dynamics of natural populations.


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