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Light- and phosphorylation-induced conformational change in phytochrome A and photoinduced electron transfer from stentorin
Abstract
Phytochromes are ubiquitous red/far-red wavelength-sensitive photoreceptors in plants. Dynamic quenching of the two component tryptophan fluorescence of pea phytochrome A has revealed differential accessibility of certain residues. Both acrylamide and Tl$\sp+$ ions showed preferential exposure of some tryptophans in Pfr-phytochrome. The Pr short lifetime component was more accessible to Cs$\sp+;$ the long component accessibility was 2 fold higher in Pfr. The absence of the fluorescence contributions of 3 residues which reacted with 2-Hydroxy-5-nitrobenzyl bromide (HNB-Br) in both photoisomers increased the Tl$\sp+$ $K\sb{\rm sv}$'s for Pr and Pfr. The 2 HNB-Br modifications specific for Pfr resulted in a reversal of the Stern-Volmer plots relative to the unmodified protein. Therefore, topographic changes associated with the phytochrome phototransformation are not confined to the 58 kDa chromophore domain. Phytochrome A is a phosphoprotein. To assess the modulation of phytochrome structure by phosphorylation/dephosphorylation the conformations of oat phyA have been probed by PKA catalyzed phosphorylation. No significant changes in secondary structure of the phyA molecule after its phosphorylation were observed by CD. A subtle topographic and/or electrostatic effect of the phytochrome phosphorylation was detected by the fluorescence quenching of tryptophans with Cs$\sp+$ ions. The ability of hypericin and stentorin to act as electron donors was also studied. The observed correlation between free energy change of electron transfer and quenching rate constant suggests that quenching proceeds via electron transfer from hypericin and stentorin to the quenchers. Their EPR spectra demonstrated that free radical formation was initiated or enhanced by visible light and that similar radical species were produced in each sample. The EPR signal for stentorin was significantly enhanced by 1,4-benzoquinone. We suggest that electron transfer in the excited state of these chromophores results in the formation of a cation radical.
Subject Area
Biochemistry|Biophysics
Recommended Citation
Wells, Todd Alan, "Light- and phosphorylation-induced conformational change in phytochrome A and photoinduced electron transfer from stentorin" (1997). ETD collection for University of Nebraska-Lincoln. AAI9736958.
https://digitalcommons.unl.edu/dissertations/AAI9736958