ArsD is a weak autogenous represser of the R773 ars operon. Its physiological role is to limit expression of the operon when the levels the Ars proteins reach a concentration above which the amount of ArsB, a very hydrophobic membrane protein, becomes toxic. The 13 kDa ArsD protein was purified. From its elution on gel filtration chromatography purified ArsD appears to be a 26 kDa homodimer in solution. By DNase I footprinting, ArsD was shown to bind to the same sequence in the promoter region as the ArsR represser. In mobility shift assays ArsD retarded the migration of a DNA fragment containing the operator/promo ter sequence, and arsenite resulted in dissociation of ArsD from the DNA. However retardation required considerably higher concentrations nf ArsD than ArsR.. consistent with ArsD beinp a relative weak renressor required only to limit expression of ArsB.
