The modABC operon of phytopathogen Xanthomonas axonopodis pv. citri (X. citri) encodes a putative ABC transporter involved in the uptake of the molybdate and tungstate anions. Sequence analyses showed high similarity values of ModA orthologs found in X. campestris pv. campestris (X. campestris) and Escherichia coli. The X. citri modA gene was cloned :in pET28a and the recombinant protein, expressed in the E. coli BL21 (DE3) strain, purified by immobilized metal affinity chromatograplay. The purified protein remained soluble and specifically bound molybdate and tungstate with K-d 0.29 +/- 0.12 mu M and 0.58 +/-0.14 mu M, respectively. Additionally binding of molybdate drastically enhanced the thermal stability of the recombinant ModA as compared to the apoprotein. This is the first characterization of a ModA ortholog expressed by a phytopathogen and represents an important tool for functional, biochemical and structural analyses of molybdate transport in Xanthomonas species. (c) 2006 Elsevier Inc. All rights reserved.