| Summary: | 碩士 === 長庚大學 === 基礎醫學研究所 === 95 === A cad operon that is related to cadmium and zinc resistance was recently identified from Streptococcus salivirius 57.I chromosome. The cad operon contains a functional gene, cadD, and a regulatory gene, cadX. The result of sequence analysis revealed that the cadD of S. salivirius 57.I was homologous to the cadB-like gene on plasmid pLUG10 from Staphylococcus lugdunensis, and cadD on many Staphylococcus aureus plasmids. The cadX of S. salivirius 57.I also shred significant homology with the cadD on plasmid pI258 from S. aureus. To determine whether this cad operon was involved in heavy metal resistance in S. salivirius 57.I, a cadD-deficient and a cadX-deficient mutant strain were generated by insertion inactivation, and the minimal inhibitory concentrations (MICs) to cadmium and zinc were determined in the wild-type and mutant strains. Comparing to wild-type S. salivirius, MICs of cadmium and zinc decreased 6 and 1.6-fold in the cadD-deficient strain, respectively. In contract, MICs of these two metals in the cadX-deficient strain increased 5 and 6.25-fold, respectively,. Furthermore, by using promoter-cat reporter gene fusion technique we found that the expression of cad promoter was repressed by CadX. However, this phenomenon was relieved in the presence of cadmium. In conclusion, the cad operon of S. salivirius 57.I is an inducible system that confers cadmium and zinc resistance. The expression of cad operon is negatively regulated by CadX at the transcriptional level, and the induction occurs by the presence of cadmium.
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