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The manganese-responsive repressor Mur of Rhizobium leguminosarum is a member of the Fur-superfamily that recognizes an unusual operator sequence

¹ School of Biological Science, University of East Anglia, Norwich NR4 7TJ, UK
² Department of Molecular Microbiology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK

Correspondence
R. Gary Sawers
gary.sawers{at}bbsrc.ac.uk

The manganese uptake regulator Mur of Rhizobium leguminosarum is a close homologue of the global iron regulatory protein Fur. Mur represses the sitABCD operon, which encodes a Mn²⁺ transport system, specifically in response to Mn²⁺ but not Fe²⁺. In previous work the authors mapped the 5' ends of two sit operon transcripts, termed TS1 and TS2, which were co-ordinately regulated by Mn²⁺-Mur, but this paper now shows that only TS1 is a primary transcript. DNase I protection analyses showed that purified Mur bound, with similar affinity, to two sites in the regulatory region of sitABCD, but only when Mn²⁺ was present in the reaction buffer. These Mn²⁺-Mur-binding sites, termed MRS1 and MRS2 (Mur-responsive sequence), were closely related in sequence to each other and were separated by 16 bp, spanning the transcription initiation site TS1. The extent of the protected DNA was 34 and 31 bp for MRS1 and MRS2, respectively, which is in accord with other members of the Fur family. The DNA sequences recognized by Mn²⁺-Mur are wholly different from conventional Fur boxes, but some similarities to a recognition sequence for the Fur regulator from Bradyrhizobium japonicum were noted. Transcription analysis of the R. leguminosarum mur gene showed its expression to be independent of Mn²⁺-Mur. Thus, Mur is a sequence-specific DNA-binding protein that responds in vitro to manganese, and thus can occlude RNA polymerase access to the sitABCD promoter. Moreover, Mur recognizes a DNA sequence atypical for the Fur superfamily and, like Fur from B. japonicum, defines a new subclass of Fur-like transcriptional regulators.

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