Figure 1.

Comparison of Smc complexes and their possible modes of action on chromosomes. (A) Cartoon of the subunit composition of the bacterial Smc complex, its binding sites along the circular bacterial chromosome, and a proposal of how it might compact the chromosome by establishing links between neighbouring binding sites. The movement of bacterial replication products towards opposite cell poles initiates from the parS partitioning locus, which is enriched in Smc complexes. Additional binding sites along the chromosome arms correlate with highly expressed genes, such as rRNA (rrn) and tRNA (trn). (B) The eukaryotic condensin complex has a chromosomal association pattern reminiscent of the bacterial Smc complex. It is enriched at the centromere (CEN) and at highly transcribed loci along the chromosome arms, including rRNA, tRNA and ribosomal protein genes (RPS and RPL). Condensin may similarly promote chromosome compaction by establishing interactions between neighbouring binding sites. The enrichment around the centromere provides additional mechanical stability around this locus during chromosome segregation. (C) Cohesin associates with chromosomes at the same loci as condensin, but shows pronounced translocation on loading in response to transcription, perhaps because of its longer mean residence time on chromosomes. Cohesin also establishes interactions between neighbouring binding sites as part of transcriptional insulators but has specialized in establishing stable interactions between sister chromatids during DNA replication. Brn, barren; Pds, precocious dissociation of sister chromatids; rRNA, ribosmal RNA; Scc, sister chromatid cohesion; Scp, segregation and condensation protein; Smc, structural maintenance of chromosomes; tRNA, transfer RNA; Wapl, wings apart-like; Ycg, yeast CapG; Ycs, yeast condensin subunit.