Structural Basis of the Hg2+-Mediated Conformational Switching of the Dual-Function Transcriptional Regulator MerR

• Main Authors: Chih-Chiang Chang, 張志強
• Other Authors: 詹迺立
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/90915840366893318218

博士 === 國立臺灣大學 === 生物化學暨分子生物學研究所 === 103 === The mer operon confers bacterial resistance to environmental inorganic mercury (Hg2+) and organomercurial compounds by encoding proteins involved in the sensing, transport, and detoxification of these cytotoxic agents. Expression of the mer operon is tightly regulated by the dual-function transcriptional regulatory protein MerR. Whereas in the absence of Hg2+, MerR binds to the operator/promoter region (O/P) of mer operon to supress transcription, MerR is converted into a transcriptional activator upon Hg2+-binding to induce mer operon expression. Sequence analysis suggests that the O/P of mer operon is pseudopalindromic with the -35 and -10 boxes being spaced by 19~20 bps, deviate from the optimal spacing of 17 bp. Therefore, a Hg2+-dependent DNA distortion by the MerR dimer, which brings closer and reorients the two polymerase binding sites, is required to activate transcription. To understand the structural basis by which Hg2+-binding modulates MerR function, we have determined the crystal structures of apo- and Hg2+-bound MerR dimer form Bacillus megaterium MB1, which correspond to the suppressor and activator conformation of MerR, respectively. To our knowledge, the apo-MerR structure represents the first visualization of an inducer-free form of a MerR family protein. Structural comparison not only illustrated how a buried trigonal planar Hg2+-binding pocket is assembled, but also revealed functionally relevant tertiary and quaternary changes between the apo- and Hg2+-bound MerR dimer. The pronounced Hg2+-dependent reposition of the DNA-binding domains suggests a plausible mechanism of transcription regulation by MerR.