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In wild-type Rhizobium leguminosarum, the sitABCD operon specifies a Mn(2+) transporter whose expression is severely reduced in cells grown in the presence of this metal. Mutations in the R. leguminosarum gene, mur (manganese uptake regulator), whose product resembles the Fur transcriptional regulator, cause high-level expression of sitABCD in the presence of Mn(2+). In gel-shift mobility assays, purified R. leguminosarum Mur protein bound to at least two regions near the sitABCD promoter region, although this DNA has no conventional consensus Fur-binding sequences (fur boxes). Thus, in contrast to gamma-proteobacteria, where Fur binds Fe(2+), the R. leguminosarum Fur homologue, Mur, act as a Mn(2)-responsive transcriptional regulator.
Díaz-Mireles et al. (Sat,) studied this question.