Kinetic Study of the Avocado Sunblotch Viroid Self-Cleavage Reaction Reveals Compensatory Effects between High-Pressure and High-Temperature: Implications for Origins of Life on Earth †

SIMPLE SUMMARY: Viroids remain the smallest infectious agents ever discovered. They are found in plants and consist of single-stranded non-coding circular RNA. Due to their simplicity, viroids are considered relics of an ancient RNA World that may have originated in the deep seas near hydrothermal vents where temperature and pressure are both elevated. To test this hypothesis, a synthetic avocado sunblotch viroid, whose structure contain an autocatalytic hammerhead ribozyme, was subjected to increased pressure (from atmospheric pressure to 300 MPa) at different temperatures (0–65 °C) and the reaction rate constant of the catalytic activity was calculated for each condition. The results obtained allowed calculation of the positive activation volume of this viroid and revealed a compensatory effect between pressure and temperature. In conclusion, these results not only exemplify the plasticity of RNA and support the RNA World hypothesis, but also highlight the usefulness of the hydrostatic pressure in understanding the structure–function relationships of biomacromolecules. ABSTRACT: A high pressure apparatus allowing one to study enzyme kinetics under pressure was used to study the self-cleavage activity of the avocado sunblotch viroid. The kinetics of this reaction were determined under pressure over a range up to 300 MPa (1–3000 bar). It appears that the initial rate of this reaction decreases when pressure increases, revealing a positive ΔV≠ of activation, which correlates with the domain closure accompanying the reaction and the decrease of the surface of the viroid exposed to the solvent. Although, as expected, temperature increases the rate of the reaction whose energy of activation was determined, it appeared that it does not significantly influence the ΔV≠ of activation and that pressure does not influence the energy of activation. These results provide information about the structural aspects or this self-cleavage reaction, which is involved in the process of maturation of this viroid. The behavior of ASBVd results from the involvement of the hammerhead ribozyme present at its catalytic domain, indeed a structural motif is very widespread in the ancient and current RNA world.