Single GAF Domain Phytochrome Exhibits a pH-Dependent Shunt on the Millisecond Timescale.

The light-sensing activity of phytochromes is based on the reversible light-induced switching between two isomerization states of the bilin chromophore. These photo-transformations may not necessarily be only unidirectional, but could potentially branch back to the initial ground state in a thermally driven process termed shunt. Such shunts have been rarely reported, and thus our understanding of this process and its governing factors are limited. Here, we aim to close this gap by providing coherent experimental evidence of a shunt process using UV/Vis laser flash photolysis. We studied the Pfr to Pr dynamics of the single GAF domain (g1) construct of the knotless phytochrome All2699 from cyanobacterium Nostoc punctiforme. We identified a shunt that can be switched on and off by ambient buffer conditions. In combination with H/D exchange and kinetic modeling, we propose a keto-enol tautomerism to allow for the thermal isomerization of the chromophore and act as the driver of the shunt transition.