Light-Dependent Tyrosine Phosphorylation in the Cyanobacterium Prochlorothrix hollandica

Abstract

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. lhis 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. l h e 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 pmol photons m-' se'), the 88-kD protein was detectable upon lndia 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.

A low molecular weight protein tyrosine phosphatase from Synechocystis sp. strain PCC 6803: enzymatic characterization and identification of its potential substrates

“…It is primarily because the phosphorylation states of the proteins depend on the intensity of light at which Synechocystis sp. was grown (13,15). In our studies, Synechocystis was grown at low light intensity, 10 mmol photons m À2 s À1 , a condition that is not conducive for elevated levels of tyrosine phosphorylation.…”

On the Extent of Tyrosine Phosphorylation in Chloroplasts

“…phosphorylation was also reported for cyanobacteria (Warner and Bullerjahn, 1994), and a dualspecificity kinase was identified in tobacco (Nicotiana tabacum) chloroplasts (Cho et al, 2001). Thus, there are several reasons why it is possible or even likely that chloroplasts use Tyr phosphorylation in their signaling; however, our search for clear-cut evidence for Arabidopsis chloroplast proteins was unsuccessful, and the putative targets for Tyr phosphorylation remain elusive.…”

Fancy meeting you here! A fresh look at "prokaryotic" protein phosphorylation

“…Although possibly coincidental, this correlation raises the possibility that a phosphotyrosinecontaining protein that is ubiquitous in distribution and function among bacteria exists. It is also noteworthy that in the cyanobacterium Prochlorothrix hollandica, the degree of phosphorylation of the 88-kDa phosphotyrosine-containing protein was observed to be light dependent (49). In N. commune UTEX 584, the appearance of the 85-kDa tyrosine-phosphorylated protein was dependent upon the presence of fixed nitrogen (38).…”