POINT MUTATIONS IN THE CHLOROPLAST 16S RIBOSOMAL-RNA GENE CONFER STREPTOMYCIN RESISTANCE IN NICOTIANA-PLUMBAGINIFOLIA

In a previous paper we reported the isolation of streptomycin-resistan t mutants from Nicotiana plumbaginifolia and presented evidence for ch loroplast control of the resistance trait. To understand the molecular basis of the resistance in these mutants, we sequenced three regions in the chloroplast 16s rRNA gene, which correspond to the 5' terminus, the 530 loop, and the 900 stem/loop of Escherichia coli 16s rRNA, and compared them with the sequences of the wild-type. Our results show t hat: (1) nine mutants have a C to T change at position 912, (2) one mu tant (SR1021) has a G to A change at position 885, (3) one mutant has a C to T change at position 526, based on E. coli numbering; and (4) t hree mutants do not have any change in the regions analyzed. The point mutation detected in SR1021 has not been reported previously. In E. c oli 16s rRNA, position 885 is protected from chemical probing by ribos omal protein S12 and is closely juxtaposed with the streptomycin-bindi ng region (positions 912-915) in the predicted secondary structure. It is likely that the G to A transition at this position is a novel muta tion for streptomycin resistance.