Variant clones resistant to 40 μg/ml chloramphenicol were isolated from the human cell line VA
2-B after treatment with either ethyl methanesulfonate or N-methyl-N′-nitro-N-nitrosoguanidine. Among 17 clones analyzed, one variant, CAP-23, was investigated in detail.
CAP-23 cells in the presence of 40 or 100 μg/ml chloramphenicol grew at essentially the same rate as cells in the absence of the drug; chloramphenicol resistance persisted even after 20 generations in the absence of the drug. No obvious morphological changes in mitochondria were observed by electron microscopy of thin sections of CAP-23 cells. In vivo mitochondrial protein synthesis in CAP-23 cells was inhibited little, if any, by chloramphenicol, and the variant showed partial cross resistance to mikamycin and carbomycin. In vitro protein synthesis in mitochondria isolated from CAP-23 cells showed, likewise, low levels of inhibition by chloramphenicol. This suggests that the drug resistance of the variant CAP-23 is due to altered mitochondria.