Coupling chromosome organization to genome segregation in Archaea

Chromosome segregation is a fundamental process in all life forms and requires coordination with genome organization, replication and cell division. The mechanism that mediates chromosome segregation in archaea remains enigmatic, despite the centre-stage role assumed by these organisms in the discourse about the origin of eukaryotes. Previously, we identified two proteins, SegA and SegB, which form a minimalist chromosome partition machine in Sulfolobales. Here we uncover patterns and mechanisms that the SegAB system employs to link chromosome organization to genome segregation. Deletion of the genes causes growth and chromosome partition defects. ChIP-seq investigations revealed that SegB binds to multiple sites scattered across the chromosome, but mainly localised close to the segAB locus in most of the examined archaeal genera. The sites are predominantly present in intragenic regions. Recent chromosome conformation studies have shown that the chromosome of Sulfolobales members is organised into two spatially segregated compartments, designated A and B. Interestingly, SegB sites are predominantly located in the A compartment, whose shaping factors have remained elusive so far. We show that SegB coalesces into multiple foci through the nucleoid, exhibiting a biased localisation towards the cell periphery, which hints at potential tethers to the cell membrane. This clue is further strengthened by the structural similarities of SegB orthologues with predicted b-helix domains to membrane-associate proteins, including the distant bactofilin. Atomic force microscopy experiments provided mechanistic insights into how SegB binds DNA and uncovered short-range DNA compaction and long-range looping of distant sites by SegB. These activities point to a significant role for SegB in chromosome condensation that in turn enables genome segregation. Collectively, our data put forward SegAB as important players in bridging chromosome organization to genome segregation in archaea. ChIP-seq of exponential phase S. acidocaldarius cells in wild type versus ΔsegB strains, using anti-SegB polyclonal antibody