Cloning and characterisation of malE in Burkholderia pseudomallei

No recombinant protein is available for serodiagnosis or skin test in the diagnosis of melioidosis. This report describes the cloning of the malE gene, which encodes an immunogenic protein of Burkholderia pseudomallei. Bi-directional DNA sequencing of malE revealed that the gene contained a single open reading frame encoding 416 amino acid residues with a predicted molecular mass of 44.4 kDa. BLAST analysis showed that the putative protein encoded by malE is homologous to the maltose-binding protein (MBP) of other bacteria. It has 48% and 63% amino acid identity and similarity with the MBP of Brucella abortus, and malE complementation assay showed that it partially complemented the function of the MBP of Escherichia coli. Several highly conserved regions among the MBP of B. pseudomallei, Br. abortus, Salmonella enterica serotype Typhimurium, E. coli and Enterobacter aerogenes were observed. These regions represent signatures A, B, C, D and F identified in the MBP of E. coli. Further sequence analysis revealed that the first 24 amino acid residues of the MBP of B. pseudomallei probably represent the N-terminal signal peptide of the protein. Similar to the signal peptide of the MBP of E. coli, Ent. aerogenes and S. Typhimurium, the MBP of B. pseudomallei contains two basic residues in the first eight amino acids, followed by a hydrophobic core, with the last three amino acids in the signal peptide being Ala-Gln-Ala, conforming to the consensus sequence Ala-X-Ala at positions -3 to -1 relative to the site of proteolytic cleavage for recognition by signal peptidase I. Further studies on serodiagnosis of melioidosis with recombinant MBP should be performed.