Structure of Mycobacterium tuberculosis single-stranded DNA-binding protein. Variability in quaternary structure and its implications

K. Saikrishnan,^a J. Jeyakanthan,^a J. Venkatesh,^b N. Acharya,^b K. Sekar,^c U. Varshney,^b and M. Vijayan^a

^aMolecular Biophysics Unit; ^bDepartment of Microbiology and Cell Biology; ^cBioinformatics Centre, Indian Institute of Science, Bangalore, 560012, India (ksai@mbu.iisc.ernet.in)

Single-stranded DNA-binding protein (SSB) is an essential protein necessary for the functioning of the DNA replication, repair and recombination machineries. The protein binds preferentially to single-stranded DNA (ssDNA). As part of tuberculosis structural genomics project with special emphasis on repair and recombination, we have solved the structure of the DNA-binding domain of Mycobacterium tuberculosis SSB (MtuSSB) in four different crystals distributed in two forms. The structure of one of the forms was solved by a combination of isomorphous replacement and anomalous scattering. This structure was used to determine the structure of the other form by molecular replacement. Like most other SSBs, topologically the DNA-binding domain contains the OB-fold. The globular core of the molecule in different subunits in the two forms and those in Escherichia coli SSB (EcoSSB) and human mitochondrial SSB (HMtSSB) have similar structure, although the three loops exhibit considerable structural variation. However, the tetrameric MtuSSB has an as yet unobserved quaternary association. This quaternary structure with a unique dimeric interface lends the oligomeric protein greater stability, which may be of significance to the functioning of the protein under conditions of stress. Modelling studies indicate a difference in the path adopted by the ssDNA to wrap around MtuSSB, in comparison to EcoSSB-ssDNA.