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Spectinomycin is an aminoglycoside antibiotic that inhibits prokaryotic-like protein synthesis by blocking translocation of messenger RNA and transfer RNAs on the ribosome. Mutants resistant to this antibiotic have been isolated from a wide range of bacteria, the alga Chlamydomonas and from chloroplasts of different species of higher plants. Usually a single nucleotide substitution occurred on chloroplast 16S ribosome RNA gene is sufficient to resist spectinomycin. Mutation rates of 16S rRNA gene of Nicotiana tabacum L treated with different concentrations of spectinomycin were studied. The results showed a sharp decrease of green resistant shoots from 30 to 300 mg l -1 spectinomycin, then an increase from 300 to 500 mg l -1 and finally a gradual decrease from 500 to 1000 mg l -1 . The increasing number of green resistant shoots from 300 to 500 mg l -1 spectinomycin indicates that the higher selective pressure applied by the antibiotic promoted a faster evolutionary rate of 16S rRNA gene. Furthermore, a new substitution site of 16S rDNA which conferred resistance to spectinomycin was isolated and confirmed by sequencing, endorsing that mutation is due to selection through high concentration of the antibiotic.

Evolution of Chloroplast 16S Ribosome RNA Dependent Spectinomycin Resistance and Implications for Chloroplast Transformation