Cytoplasmic transfer of chloramphenicol resistance in a human cell line

Abstract

CAP-23, a chloramphenicol-resistant variant of the human cell line VA ₂-B, was enucleated by two different methods, and fused, using UV-inactivated Sendai virus, with a 6-thioguanine-resistant derivative (TG-6) of the same VA ₂-B cells. In selective medium containing 100 or 200 μg/ml chloramphenicol and 10 μg/ml 6-thioguanine, resistant clones appeared with a frequency of 10 ⁻⁵ to 10⁻³ in different experiments. Control fusion mixtures (CAP-23 × TG-6; enCAP-23 × CAP-23; enTG-6 × TG-6; CAP-23 × enTG-6) and the parental cells themselves produced few or no resistant clones, thus eliminating the possibility that spontaneous mutations or fusion products other than cytoplasmic hybrids of enCAP-23 and TG-6 were responsible for the high frequency of resistant clones observed in the enCAP-23 × TG-6 fusions. Fifteen individually isolated clones were subjected to karyotype analysis and found to contain a mean chromosome number very similar to that of the parental cells. The in vitro protein synthesis in mitochondria isolated from cytoplasmic hybrid clones showed a level of resistance to chloramphenicol similar to that found in mitochondria from the parental CAP-23 cells. Thus, the present experiments show that the chloramphenicol resistance of CAP-23 cells has been transferred by a cytoplasmic (presumably mitochondrial) gene into sensitive cells.