RNAs undergo phase transitions with lower critical solution temperatures

Abstract

Co-phase separation of RNAs and RNA-binding proteins is thought to drive the biogenesis of ribonucleoprotein granules. RNAs can also undergo phase transitions in the absence of proteins. However, the physicochemical driving forces of protein-free RNA-driven phase transitions remain unclear. Here, we report that RNAs of various types undergo phase transitions with system-specific lower critical solution temperatures. This entropically-driven phase behavior requires Mg2+ions and is an intrinsic feature of the phosphate backbone that is modulated by RNA bases. RNA-only condensates can additionally undergo a temperature-dependent percolation transition, which is enabled by a combination of Mg2+-dependent bridging interactions among phosphate groups and RNA base-stacking / base-pairing. Phase separation coupled to percolation can cause dynamical arrest of RNAs within condensates. The dynamical arrest of condensates formed by the RNase P ribozyme suppresses its catalytic activity. Our work highlights the need to incorporate RNA-driven phase transitions into models for RNP granule biogenesis.

Competition between self-assembly and phase separation induces re-entrant condensation of DNA liquids

“…However, our system is distinct in that LLPS is opposed by stoichiometric complex self-assembly. Our results can also be compared with recent reports of hightemperature LLPS in RNA solutions at high MgCl 2 concentrations, which similarly noted unexpectedly strong sequence-dependent effects [21]; however, the interpretation of the mechanism governing the phase boundary in that system differs from the present work.…”

“…Nonetheless, as we show below, the precise determinants of the phase boundaries cannot be explained solely by the counter ion identity and concentration. We also note that the ability of divalent cations to condense nucleic acids at high temperatures has recently been explored in RNA solutions [21], suggesting that similar mechanisms may be involved in other systems.…”

Phase Transitions of Associative Biomacromolecules

“…For a system exhibiting a lower LCST phase behavior is entropically driven and enthalpically stabilized. 138,139 The entropic penalty associated with organizing solvent molecules around functional groups along the chain will increase with increasing temperature. This penalty is reduced by the release of solvent, and the segregation of polymers into a solvent-deficient phase.…”

Theory and Simulation of Multiphase Coexistence in Biomolecular Mixtures

“…Folded domains within multidomain proteins can also contribute to the multivalency required to drive LLPS, either through protein-protein interactions (PPIs) [29] or, in the case of RNA binding domains (RBDs), through interactions with RNA [30]. Finally, nucleic acid mixtures can phase separate under certain conditions due to intermolecular base-pairing [31][32][33] and nonspecific association [34]. Importantly, the strengths of the net interactions among biopolymers in liquid-like condensates are typically comparable to the thermal energy, since the protein and nucleic acid constituents of biomolecular condensates can often remain fluid on biologically relevant timescales.…”

“…30 Finally, nucleic acid mixtures can phase separate under certain conditions due to intermolecular base-pairing 31−33 and nonspecific association. 34 Importantly, the strengths of the net interactions among biopolymers in liquid-like condensates are typically comparable to the thermal energy, since the protein and nucleic acid constituents of biomolecular condensates can often remain fluid on biologically relevant time scales.…”