Increased kasugamycin sensitivity in Escherichia coli caused by the presence of an inducible erythromycin resistance (erm) gene of Streptococcus pyogenes.
An inducible erythromycin resistance gene (erm) of Streptococcus pyogenes was introduced into Escherichia coli by transformation with a plasmid. The recipient E. coli cells were either kasugamycin sensitive (wildtype) or kasugamycin resistant (ksgA). The MIC values of erythromycin increased from 150 micrograms/ml to greater than 3000 micrograms/ml for E. coli. An extract of transformed cells, particularly a high-salt ribosomal wash, contained an enzyme that was able to methylate 23S rRNA from untransformed cells in vitro; however, 23S rRNA from transformed cells was not a substrate for methylation by such an extract. 165 rRNA and 30S ribosomal subunits of either the wild type or a kasugamycin resistant (ksgA) mutant were not methylated in vitro. Transformation of E. coli by the erm-containing plasmid led to a reduction of the MIC values for kasugamycin. This happened in wild-type as well as in ksgA cells. However, in vitro experiments with purified ksgA encoded methylase demonstrated that also in erm transformed E. coli, the ksgA encoded enzyme was active in wild-type, but not in ksgA cells. It was also shown by in vitro experiments that ribosomes from erm ksgA cells have become sensitive to kasugamycin. Our experiments show that in vivo methylation of 23S rRNA, presumably of the adenosine at position 2058, leads to enhanced resistance to erythromycin and to reduced resistance to kasugamycin. This, together with previous data, argues for a close proximity of the two sites on the ribosome that are substrates for adenosine dimethylation.