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Abstract:
Smc–ScpAB forms elongated, annular structures that promote chromosome segrega-
tion, presumably by compacting and resolving sister DNA molecules. The mechanistic basis for its
action, however, is only poorly understood. Here, we have established a physical assay to determine
whether the binding of condensin to native chromosomes in
Bacillus subtilis
involves entrapment of
DNA by the Smc–ScpAB ring. To do so, we have chemically cross-linked the three ring interfaces in
Smc–ScpAB and thereafter isolated intact chromosomes under protein denaturing conditions.
Exclusively species of Smc–ScpA, which were previously cross-linked into covalent rings, remained
associated with chromosomal DNA. DNA entrapment is abolished by mutations that interfere with
the Smc ATPase cycle and strongly reduced when the recruitment factor ParB is deleted, implying
that most Smc–ScpAB is loaded onto the chromosome at
parS
sites near the replication origin. We
furthermore report a physical interaction between native Smc–ScpAB and chromosomal DNA
fragments
