Abstract

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.

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