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Crystal structure of single-stranded DNA-binding protein from Streptomyces coelicolor

Single stranded DNA-binding proteins (SSBs) occur in all organisms and many viruses. They play an essential role in DNA replication, recombination and repair. Whenever single-stranded DNA is formed, SSBs function to protect and sequester the single-stranded regions until the double helix can be reformed. All bactrial SSB proteins share a structural motif called oligonucleotide/oligosaccharide-binding fold (OB fold)1, the principal DNA-binding unit, typically consisting of around 100 amino acids. Full-length Streptomyces coelicolor SSB protein crystallised in space group I222 with unit-cell parameters a = 100.8, b = 102.1, c = 164.2 Å .2 The data were collected at the DND-CAT ID5 beamline at the Advance Photon Source synchrotron radiation facility in Argonne National Laboratory, Chicago. The structure was refined at 2.1 Å resolution. The structure was solved by molecular replacement method using as a model structure of Mycobacterium smegmatis SSB protein.3 The title protein crystallises as a homo-tetramer and exhibits approximate 222 non-crystallographic symmetry. Only one of the three perpendicular non-crystallographic symmetry axes is parallel to crystallographic two-fold axis along axis c. This symmetry was exploited during phase improvement and in the refinement. As in the case of the Mycobacterium SSBs, S. coelicolor protein has an additional β -strand at the C terminus of the OB fold domain. It is supposed that this additional β -strand is responsible for the unique quaternary structure in Mycobacterium SSBs as well as in this structure. To our knowledge this is the first crystal structure of SSB protein from Streptomyces.

Single stranded DNA-binding protein; crystal structure

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