Characterisation of the bacteriophage 186 immunity repressor by mass spectrometry, SAXS and crystallography reveals a dodecamer structure with flexibly attached DNA binding domains

JQ Truong1, H Mertens2, F Whelan3, T Pukala1, C Lun1, J Bruning1, IB Dodd1 and KE Shearwin1

  1. University of Adelaide, Faculty of Sciences, North Terrace, Adelaide SA 5003, Australia
  2. EMBL Hamburg, Notkestrasse 85, Hamburg, 22607, Germany
  3. University of York, Department of Biology, Heslington, York YO10 5DD, United Kingdom

The CI immunity repressor protein from bacteriophage 186 forms a wheel-like oligomer. To complement previous biophysical and in vivo reporter studies, the 186 CI protein was further studied using small angle X-ray scattering and electrospray ion mobility mass spectrometry. In addition, we have constructed a synthetic chimeric repressor protein, where the well characterised bacteriophage λ CI DNA binding domain is fused to the oligomerization domain of the 186 CI protein. The crystal structure of this chimeric repressor was solved in complex with DNA. Collectively, the results show that repressor dimers undergo further sequential oligomerization to form a dodecameric wheel. The DNA binding domains are shown to be connected to the oligomerization domains via a flexible linker which is thought to contribute to the protein's ability to bind to operators with different sequences and variable spacing. This complex becomes a scaffold that DNA can wrap around and loop on and off, to selectively control gene expression.