Design principle of most bacteria sensor–reporters for arsenic. a When no arsenic enters the cell, the ArsR protein represses the transcription of the arsenic defense system genes (arsD, arsC, arsA, and arsB) from one particular DNA region upstream of the gene for itself (the operator–promoter site). In the presence of arsenite in the cell, ArsR loses affinity for the operator and RNA polymerase will transcribe the arsDCAB genes to produce the defense. ArsC is a reductase that reduces arsenate [(As(V)] to arsenite [As(III)], whereas ArsAB constitute an efflux pump for arsenite. b In the sensor–reporter strain, an extra copy of the operator–promoter DNA fused to the arsR gene and a gene for a reporter protein are added to the cell. In this case, when arsenite or arsenate is sensed by the cell, transcription for the reporter gene will also be unleashed and the reporter protein will be formed. The presence or activity of the reporter protein is subsequently measured
The way that this protein can invoke this response is by its action
as a transcriptional repressor, which, in the absence of arsenic,
binds to a specific DNA sequence (i.e., the operator) overlapping
with the binding site for RNA polymerase (the promoter;
To turn this natural defense system into a workable bioassay,
the bacterial cell is equipped with a second copy of the
operator–promoter sequence for ArsR, which is now transcriptionally
fused to a gene for a so-called reporter protein (Fig. 1).
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