Genetic Basis of Chloramphenicol Resistance in Mouse and Human Cell Lines

Abstract

Chloramphenicol resistance in mammalian cells is cytoplasmically inherited (Bunn et al. 1974). In yeast, sequencing of mtDNA from strains with different alleles at the rib1 locus has identified nucleotide monosubstitutions in the large-rRNA gene which correlate with resistance to the antibiotic (Dujon 1980). By sequencing the corresponding region of mammalian mtDNA from chloramphenicol-resistant cell lines, we have found differences from the wild-type sequence at positions similar to those altered in the yeast mutants.

DISCUSSION
The chloramphenicol-resistant mammalian cell lines have a single-base change in the relevant region of the large-rRNA gene, compared with the parental cell lines (Fig. 1). mtDNA from the mouse cell line 3T3CAP^R has a base substitution close to the homologous site of the yeast cap^R323 mutation, whereas that from the human cell line MC63 has a substitution close to the yeast cap^R321 mutation. This correlation suggests that these alterations are responsible for the cellular phenotype. These substitutions all occur in sequence blocks that are identical in E. coli and in the yeast mitochondrion (Fig. 1); hence, it is unlikely that neutral polymorphisms are being detected. Such conservation probably reflects functional constraints imposed on the rRNA and partly explains why a small change has a considerable effect on the phenotype of the cell.

Kethoxal-modification studies on the E. coli ribosome have indicated that the part of the rRNA in which the chloramphenicol-resistance loci occur is at the interface between the ribosomal subunits (Herr and Noller 1979). This agrees well with the high degree of sequence conservation around...