Heterotypic phase separation of Hfq is linked to its roles as an RNA chaperone

Hfq, an Sm-like protein and the major RNA chaperone in E. coli, has been shown to distribute non-uniformly along a helical path under normal growth conditions and to relocate to the cell poles under certain stress conditions. We have previously shown that Hfq relocation to the poles is accompanied by polar accumulation of most small RNAs (sRNAs). Here, we show that Hfq undergoes RNA-dependent phase separation to form cytoplasmic or polar condensates of different density under normal and stress conditions, respectively. Purified Hfq forms droplets in the presence of crowding agents or RNA, indicating that its condensation is via heterotypic interactions. Stress-induced relocation of Hfq condensates and sRNAs to the poles depends on the pole-localizer TmaR. Phase separation of Hfq correlates with its ability to perform its posttranscriptional roles as sRNA-stabilizer and sRNA-mRNA matchmaker. Our study offers a spatiotemporal mechanism for sRNA-mediated regulation in response to environmental changes.