LIGHT-DEPENDENT TYROSINE PHOSPHORYLATION IN THE CYANOBACTERIUM PROCHLOROTHRIX-HOLLANDICA

A light-dependent tyrosine kinase activity is present in soluble extra cts from the cyanobacterium Prochlorothrix hollandica. The substrate o f this tyrosine kinase activity is a soluble 88-kD protein that is pho sphorylated when cultures of P. hollandica are adapted to high-light c onditions. This phosphoprotein was identified by probing western blots of P-32-labeled soluble proteins from P. hollandica with an antibody specific for phosphotyrosine. This specificity was confirmed by compet ition experiments in which the antibody binding was abolished complete ly in the presence of excess phosphotyrosine but not phosphoserine and phosphothreonine. The kinetics of phosphorylation in vivo were determ ined 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 phosphorlyated form of this polypeptide. Within 6 h of a downshift from high to low l ight, the 88-kD protein was dephosphorylated. In vitro phosphorylation studies also showed that cell extracts can phophorylate a tyrosine-co ntaining artificial substrate; acid hydrolysis of both the artificial substrate and the 88-kD protein showed that phosphorylation occurred e xclusively on tyrosine residues. Finally, experiments with high-light- adapted Synechococcus sp. PCC7942 suggest that a similar tyrosine phos phorylation event occurs in a phycobilisome-containing cyanobacterium.