Integration Host Factor (IHF) bends DNA. This is established textbook biology. IHF binds to a specific DNA sequence and introduces a sharp bend — approximately 160 degrees — that enables promoter activation, recombination, and chromosome compaction in bacteria. The protein is small (a heterodimer of two subunits), well-characterized structurally, and understood to perform a single mechanical function: it bends.
Parthasarathy, Ramakrishnan, and colleagues found that at low pH — below 5, the conditions characteristic of bacterial biofilms — IHF stops bending DNA and starts bridging it. The same protein, in the same organism, performing the opposite mechanical function.
The mechanism is protonation. At neutral pH, certain residues on the protein surface carry neutral charge. As the pH drops, histidine residues become protonated, exposing positively charged patches on the protein's exterior. These patches attract DNA from neighboring strands, and IHF transitions from an intramolecular bender (reshaping one DNA strand) to an intermolecular crosslinker (connecting separate DNA strands into a meshwork). The structural change in the protein is minimal — no refolding, no conformational switch in the usual sense. The charged landscape on the surface shifts, and that surface change rewrites the protein's mechanical identity.
In biofilms, the pH gradient is steep. Surface layers are near-neutral; deep layers are acidic from fermentation products. IHF at the surface bends DNA — performing its gene-regulatory role. IHF in the interior crosslinks DNA — contributing to the structural integrity of the extracellular matrix. Same protein, same genome, different function, separated by a pH gradient of two units.
The broader pattern: function is not encoded in structure alone. It is encoded in structure-plus-environment. A molecule that “is” a bender under one condition “is” a crosslinker under another. The identity of the protein is not a fixed property — it is a relational property between the molecule and its context. Change the context by two pH units and the same machine does the opposite job.