ATP-dependent aggregation of single-stranded DNA by a bacterial SMC homodimer

SMC ((s) under bar tructural (m) under bar aintenance of (c) under bar hrom osomes) proteins are putative ATPases that are highly conserved among Bacte ria, Archaea and Eucarya, Eukaryotic SMC proteins are implicated in a diver se range of chromosome dynamics including chromosome condensation, dosage c ompensation and recombinational repair. In eukaryotes, two different SMC pr oteins form a heterodimer, which in turn acts as the core component of a la rge protein complex. Despite recent progress, no ATP-dependent activity has been found in individual SMC subunits. We report here the first biochemica l characterization of a bacterial SMC protein from Bacillus subtilis. Unlik e eukaryotic versions, the B. subtilis SMC protein (BsSMC) is a simple homo dimer with no associated subunits. It binds preferentially to single-strand ed DNA (ssDNA) and has a ssDNA-stimulated ATPase activity. In the presence of ATP, BsSMC forms large nucleoprotein aggregates in a ssDNA-specific mann er. Proteolytic cleavage of BsSMC is changed upon binding to ATP and ssDNA. The energy-dependent aggregation of ssDNA might represent a primitive type of chromosome condensation that occurs during segregation of bacterial chr omosomes.