Binding of AraC-Type Activator DesR to the Promoter Region of Vibrio vulnificus Ferrioxamine B Receptor Gene

Vibrio vulnificus can utilize the xenosiderophore desferrioxamine B (DFOB) as an iron source under iron-restricted conditions. We previously identified in V. vulnificus that transcription of the desA gene encoding the outer membrane receptor for ferrioxamine B (FOXB) is activated by the AraC-type transcriptional regulator encoded by desR together with DFOB. In this study, we overexpressed and purified DesR as a glutathione S-transferase-fused protein and examined interaction between the promoter region of desA and DesR. Electrophoretic mobility shift assay (EMSA) revealed that DesR directly binds to the regulatory region of desA, and this binding was enhanced by the presence of DFOB in a concentration-dependent manner, while the presence of FOXB did not affect the potentiation of their binding. Moreover, EMSA identified that DNA fragments lacking a probable DesR binding sequence were unable to form complexes with DesR. Finally, deoxyribonuclease I footprinting assay demonstrated that the DNA binding sequence of DesR is located between -27 and -50 nucleotides upstream of the desA transcription start site. These results strongly indicate that DesR can directly activate the transcription of desA in cooperation with DFOB, which acts as a coactivator for DesR.