The Agrobacterium tumefaciens Transcription Factor BlcR Is Regulated via Oligomerization

The Agrobacterium tumefaciens BlcR is a member of the emerging isocitrate lyase transcription regulators (IclR)that negatively regulates metabolism of ã-butyrolactone, and its repressing function is relieved by succinate semialdehyde (SSA). Our crystal structure showed that BlcR folded into the DNA- and SSA-binding domains, and dimerized via the DNA-binding domains. Mutational analysis identified residues, including F147, that are important for SSA association; BlcRF147A existed as tetramer. Two BlcR dimers bound to target DNA and in a cooperative manner, and distance between the two BlcR-binding sequences in DNA was critical for BlcR-DNA association. Tetrameric BlcRF147A retained DNA-binding activity, and importantly this activity was not affected by distance separating the BlcR-binding sequences in DNA. SSA did not dissociate tetrameric BlcRF147A or BlcRF147A-DNA. Besides in the SSA-binding site, F147 is located in a structurally flexible loop that may be involved in BlcR oligomerization. We propose that SSA regulates BlcR DNAbinding function via oligomerization.