PLANT PHYSIOLOGY , Vol 105, Issue 2 629-633, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Light-Dependent Tyrosine Phosphorylation in the Cyanobacterium Prochlorothrix hollandica
K. M. Warner and G. S. Bullerjahn
Department of Biological Sciences, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403-0212
A light-dependent tyrosine kinase activity is present in soluble extracts
from the cyanobacterium Prochlorothrix hollandica. The substrate of this
tyrosine kinase activity is a soluble 88-kD protein that is phosphorylated
when cultures of P. hollandica are adapted to high-light conditions. This
phosphoprotein was identified by probing western blots of 32P-labeled
soluble proteins from P. hollandica with an antibody specific for
phosphotyrosine. This specificity was confirmed by competition experiments
in which the antibody binding was abolished completely in the presence of
excess phosphotyrosine but not phosphoserine and phosphothreonine. The
kinetics of phosphorylation in vivo were determined by probing western
blots with this antibody. Within 1 h following a switch from extended
darkness to high light (200 [mu]mol photons m-2 s-1), the 88-kD protein was
detectable upon India ink staining of western blots. After 3 h, the
antibody recognized the phosphorylated form of this polypeptide. Within 6 h
of a downshift from high to low light, the 88-kD protein was
dephosphorylated. In vitro phosphorylation studies also showed that cell
extracts can phosphorylate a tyrosine-containing artificial substrate; acid
hydrolysis of both the artificial substrate and the 88-kD protein showed
that phosphorylation occurred exclusively on tyrosine residues. Finally,
experiments with high-light-adapted Synechococcus sp. PCC7942 suggest that
a similar tyrosine phosphorylation event occurs in a
phycobilisome-containing cyanobacterium.
