Summary: A new isolate of was obtained by enrichment culture for aerobic lithoautotrophic growth on CO and H. This strain, MR22, is very similar to MR11 (formerly 1b) in functioning as a donor for genetic information determining the ability to grow lithoautotrophically (Aut character) in matings with Aut strains of or closely related heterotrophic species. The strain contains a plasmid, pHG33 of about 110 kb. A mutant was isolated from strain MR22 which was plasmid-free, and had lost the Aut character, resistance to 50 μ-thallium salt and susceptibility to the nocardia-specific bacteriophage ϕBl. As a recipient of the Aut character, this plasmid-free mutant was as well suited as plasmid-bearing Aut strains of . In matings with the mutant as recipient the frequency of Aut transconjugants per donor was 3 x 10 with MR11 (pHG31-a, Aut, Tl, Str, ϕBl) and 2 x 10 with MR22 (pHG33, Aut, Tl, Str, ϕBl) as donor. Phenotypic characterization of the transconjugants, which had been selected for the Aut marker, revealed that in many cases the Aut marker had been transferred without plasmid transfer. Furthermore, plasmid-free, Aut transconjugants functioned as donors for the Aut marker. Both plasmid-free and plasmid-bearing transconjugants transferred the Aut marker to the Aut strains of with a frequency which was one or two orders of magnitude higher than that of the wild-type strains. The plasmids pHG31-a and pHG33 code for thallium resistance (50 μ-thallium acetate). The frequency of thallium-resistant transconjugants was 10 to 10 per donor; all thallium-resistant transconjugants contained the donor plasmid.

We conclude that the plasmids pHG31-a of strain MR11 and pHG33 of strain MR22 of carry the genetic information for thallium resistance but not the Aut character. As plasmid-free Aut strains can function as donors the Aut character is assumed to reside on the chromosome and to function as an independent self-transmissible genetic element.


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