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SUMMARY: Ribosomes of Streptomyces azureus, the thiostrepton producer, are resistant to the drug due to the action of an RNA-pentose methylase which acts on 23S ribosomal RNA. This enzyme, which is active in vitro on ribosomal RNA from other bacteria (e.g. Escherichia coli), employs S-adenosylmethionine as cofactor to catalyse the formation of a single residue of 2′-O-methyladenosine. The ‘thiostrepton-resistance methylase’ has been purified 11 000-fold (although not to homogeneity) and eluted from gel filtration columns with an apparent molecular weight of 35 000 to 38 000. The preferred substrate for the methylase was phenol-extracted 23S RNA. In contrast, mature 50S ribosomal subunits were unaffected by the enzyme although protein-deficient core particles, prepared by exposure of ribosomes to LiCl, retained partial substrate activity. Significantly, the methylase was inactive on the complex of 23S RNA with ribosomal protein L11.
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