Molecular Characterization of Cadmium Resistance inStreptococcus thermophilusStrain 4134: an Example of Lateral Gene Transfer

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Abstract
Two genes (cadCSt and cadASt [subscript St represents Streptococcus thermophilus]), located on the chromosome of S. thermophilus 4134, were shown to constitute a cadmium/zinc resistance cassette. The genes seem to be organized in an operon, and their transcription is cadmium dependent in vivo. The proposed product of the cadA open reading frame (CadASt) is highly similar to P-type cadmium efflux ATPases, whereas the predicted protein encoded by cadCSt (CadCSt) shows high similarity to ArsR-type regulatory proteins. The observed homologies and G+C content of this cassette and surrounding regions suggest that this DNA was derived from Lactococcus lactis and may have been introduced relatively recently into the S. thermophilus 4134 genome by a lateral gene transfer event. The complete cassette confers cadmium and zinc resistance to both S. thermophilus and L. lactis, but expression of cadASt alone is sufficient to give resistance. By using electrophoretic mobility shift assays it was shown that the CadCSt protein is a DNA binding protein that binds specifically to its own promoter region, possibly to two copies of an inverted repeat, and that this CadCSt-DNA interaction is lost in the presence of cadmium. Using lacZ fusion constructs it was shown that the cadmium-dependent expression of CadASt is mediated by the negative regulator CadCSt. A model for the regulation of the expression of cadmium resistance in S. thermophilus is discussed.