How bacterial SMC complexes organize chromosomes

Project Summary The organization and segregation of replicated chromosomes are fundamental to living systems. Structural maintenance of chromosomes (SMC) complexes play central roles in these processes in all domains of life. These ring-shaped ATPases share common structures and inter-subunit contacts, consistent with a common mechanism of action. Over the last five years, studies in Bacillus subtilis and eukaryotes have provided compelling in vivo and in vitro evidence that SMC complexes utilize ATP hydrolysis to extrude DNA loops. In the case of B. subtilis, SMC condensin complexes are loaded at centromeric parS sites near the replication origin, then translocate down the left and right chromosome arms, tethering them together. In this way, condensins generate a single chromosome loop centered on the origin that draws sister chromosomes in on themselves and away from each other. This elegantly simple loop-extrusion model provides a unifying mechanism to explain how eukaryotic SMC cohesin complexes form topologically associating domains (TADs) in interphase, how eukaryotic SMC condensin complexes compact DNA into rod-shaped sister chromatids, and how bacterial SMC condensins resolve newly replicated origins. However, this model raises an important question: how do SMC complexes extrude DNA loops when the chromosome is coated by numerous proteins and acted upon by replication and transcription machineries? And how are the topologically loaded complexes released from the chromosome? The goal of this proposed research is to understand the mechanism of condensin action in the context of cellular activities, taking advantage of the many molecular and cytological tools we have developed. First, we will determine how condensins act when they encounter the replisome or other condensin molecules. Second, we will characterize how condensins are released from the chromosome when they reach the terminus region. Finally, we will explore condensin’s role in the organization and dynamics of a multipartite bacterial genome that contains both a circular and a linear chromosome. Taken together, the proposed work has the potential to provide the general principles of chromosome folding and compaction in all organisms.