Reentrant DNA shells tune polyphosphate condensate size.

The inorganic biopolymer polyphosphate (polyP) occurs in all domains of life and affects myriad cellular processes. A longstanding observation is polyP's frequent proximity to chromatin, and, in many bacteria, its occurrence as magnesium (Mg(2+))-enriched condensates embedded in the nucleoid region, particularly in response to stress. The physical basis of the interaction between polyP, DNA and Mg(2+), and the resulting effects on the organization of the nucleoid and polyP condensates, remain poorly understood. Here, using a minimal system of polyP, Mg(2+), and DNA, we find that DNA can form shells around polyP-Mg(2+) condensates. These shells show reentrant behavior, that is, they form within a window of Mg(2+) concentrations, representing a tunable architecture with potential relevance in other multicomponent condensates. This surface association tunes condensate size and DNA morphology in a manner dependent on DNA length and concentration, even at DNA concentrations orders of magnitude lower than found in the cell. Our work also highlights the remarkable capacity of two primordial inorganic species to organize DNA.