Research Interests: The Lagarias Laboratory
Phytochrome Photochemistry
Recent crystal structures of the photosensory input regions of prokaryotic phytochromes have provided new insight into the chemical basis of phytochrome’s unique R/FR switchable photochemistry. Based on these and previous spectroscopic measurements (reviewed by Rockwell & Lagarias, 2006), the extant models for the photochemical interconversion needed revision. The forward and reverse photoconversions entail efficient Z/E isomerization of the C15-double bond of the bilin chromophore, which occur in the sub-picosecond to picosecond timescale (Figure 3). Phytochromes therefore exhibit little fluorescence, which contrast them from the intensely fluorescent phycobiliproteins of cyanobacteria, red algae and cryptophytes. The forward Pr-to-Pfr and reverse Pfr-to-Pr interconversions proceed through distinct intermediates that can be distinguished optically and phytochromes typically exhibit thermal ‘dark-reversion’ of Pfr to Pr indicating that the Pfr form is metastable.
Figure 3. Plant phytochromes are photochromic proteins that exist in two spectrally distinct forms, Pr and Pfr. Upon light absorption phytochrome interconverts between these two forms. The structure of the bilin chromophore is altered upon light absorption which gives rise to the characteristic absorption spectra of the Pr and Pfr forms.