Structural Maintenance of Chromosomes (SMC) proteins and their complex partners (ScpA and ScpB in many bacteria) are involved in chromosome compaction and segregation in all kinds of organisms. We employed single molecule tracking (SMT), tracking of chromosomal loci, and single molecule counting in Bacillus subtilis to show that in slow growing cells, ∼30 Smc dimers move throughout the chromosome in a constrained mode, while ∼60 ScpA and ScpB molecules travel together in a complex, but independently of the nucleoid. Even an Smc truncation that lacks the ATP binding head domains still scans the chromosome, highlighting the importance of coiled coil arm domains. When forming a complex, 10–15 Smc/ScpAB complexes become essentially immobile, moving slower than chromosomal loci. Contrarily, SMC-like protein RecN, which forms assemblies at DNA double strand breaks, moves faster than chromosome sites. In the absence of Smc, chromosome sites investigated were less mobile than in wild type cells, indicating that Smc contributes to chromosome dynamics. Thus, our data show that Smc/ScpAB clusters occur at several sites on the chromosome and contribute to chromosome movement.

The bacterial SMC (structural maintenance of chromosomes) complex binds nonspecifically to DNA in vitro and forms two discrete subcellular centers in vivo, one in each cell half. How this distribution is maintained is unclear. We show by time-lapse imaging of single molecules that the localization is achieved through limited, yet rapid movement of the SMC subunits through the nucleoid. Accessory ScpAB subunits mediate the arrest of 20% of SMC molecules at the center of a cell half and do not move together with the 80% mobile SMC molecules. Only free SMC, but not the preformed SMC/ScpAB complex, was able to bind to DNA in vitro, revealing distinct functions of SMC fractions. Thus, whereas SMC alone dynamically interacts with many sites on the chromosome, it forms static assemblies together with ScpAB complex partners. Our findings reveal two distinct modes of interaction of SMC with the chromosome and indicate that limited diffusion within a confined space and transient arrest may be a general mechanism for positioning proteins within a chromosome and within a noncompartmentalized cell.

dSMC and MukB complexes consist of a central SMC dimer and two essential binding partners, ScpA and ScpB (MukE and MukF), and are crucial for correct chromosome compaction and segregation. The complexes form two bipolar assemblies on the chromosome, one in each cell half. Using fluorescence recovery after photobleaching (FRAP), we provide evidence that the SMC complex has high exchange rates. This depends to a considerable degree on de novo protein synthesis, revealing that the bacterial SMC complex has high on and off rates for binding to the chromosome. A mutation in SMC that affects ATPase activity and results in exaggerated DNA binding in vitro causes a strong segregation defect in vivo and affects the localization of the entire SMC complex, which localizes to many more sites in the cell than under normal conditions. These data indicate that ATP turnover is important for the function of Bacillus subtilis SMC. In contrast, the centromere protein Spo0J and DNA gyrase showed much less exchange between distinct binding sites on the chromosome than that seen with SMC. Binding of Spo0J to the origin regions was rather static and remained partially conserved until the next cell cycle. Our experiments reveal that the SMC complex has a high, condensin-like turnover rate and that an alteration of the ATPase cycle affects SMC function in vivo, while several nucleoid-associated proteins feature limited or slow exchange between different sites on the nucleoid, which may be the basis for epigenetic-like phenomena observed in bacteria.

In all organisms, from eukaryotes to prokaryotes, the chromosome is highly compacted and organized. Chromosome condensation is essential in all cells and ranges from 1,000- to more than 10,000-fold between bacterial and eukaryotic cells. Replication and transcription occur in parallel with chromosome segregation in bacteria. Structural maintenance of chromosome proteins play a key role in chromosome compaction and segregation, their coordination with the cell cycle, and in various other chromosome dynamics, including DNA repair. In spite of their essential nature in almost all organisms, their function at a molecular level is only slowly beginning to emerge.