Accurate DNA segregation is essential for genome transmission. Segregation of the prototypical F plasmid requires the centromere-binding protein SopB, the NTPase SopA and the sopC centromere. SopB displays an intriguing range of DNA-binding properties essential for partition; it binds sopC to form a partition complex, which recruits SopA, and it also coats DNA to prevent non-specific SopA-DNA interactions, which inhibits SopA polymerization. To understand the myriad functions of SopB, we determined a series of SopB-DNA crystal structures. SopB does not distort its DNA site and our data suggest that SopB-sopC forms an extended rather than wrapped partition complex with the SopA- interacting domains aligned on one face. SopB is a multidomain protein, which like P1 ParB contains an all-helical DNA-binding domain that is flexibly attached to a compact (β3-α)2 dimer-domain. Unlike P1 ParB, the SopB dimer-domain does not bind DNA. Moreover, SopB contains a unique secondary dimer- ization motif that bridges between DNA duplexes. Both specific and non-specific SopB-DNA bridging structures were observed. This DNA-linking function suggests a novel mechanism for in trans DNA spreading by SopB, explaining how it might mask DNA to prevent DNA-mediated inhibition of SopA polymerization.
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