A Eukaryotic-Like Interaction of Soluble Cyanobacterial Sensory Rhodopsin Transducer with DNA

초록

Anabaena sensory rhodopsin is a recently discovered membrane photosensor with a unique signal transduction cascade. It interacts with a soluble tetrameric transducer [Anabaena sensory rhodopsin transducer (ASRT)] that can bind to promoter regions of several genes related to the utilization of light energy. Even though the X-ray crystal structure of ASRT is available, the mechanism of its interaction with DNA is still unknown. We used solution NMR to understand the mechanism of the DNA binding. Both Xray crystal structures and solution NMR data reveal seven beta-strands forming a rigid scaffold (beta-face) and a flexible, partially disordered alpha-face, comprised by the C-termini and loops. We found that the conformation of the alpha-face in solution is very different from that in the crystals. While the C-termini of crystalline ASRT are solvent exposed and either alpha-helical or disordered, about half of ASRT monomers in solution feature buried C-terminal beta-strand, with another half of C-tails being random coils. Titration of ASRT with a 20-bp fragment of the pec operon promoter showed that only monomers with beta-structured C-tails bind the DNA. NMR signals suggest that specific Arg and Asn/Gln residues are involved in the interaction with DNA. The DNA binding occurs with micromolar affinity and a 1:1 stoichiometry (DNA:ASRT tetramer) and results in a significant ordering of the alpha-face involving the extension of the C-terminal beta-strand and reorganization of the first loop. Such induced-fit type of interaction, which mainly utilizes loops between beta-strands and results in the increase in their order, is typical for eukaryotic transcription factors of the immunoglobulin-like fold. (C) 2011 Elsevier Ltd. All rights reserved.

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