The long-term response (LTR) to light-quality gradients improves performance and survival of plants in dense stands.It involves redox-controlled transcriptional regulation of the plastome-encoded genes psaAB (encoding the P700 apoproteins of photosystem I) and psbA (encoding the D1 protein of photosystem II) and requires the action of plastid-localized kinases.To study the potential impact of phosphorylation events on plastid gene expression during the LTR,we analyzed mustard seedlings acclimated to light sources favoring either photosystem I or photosystem II.Primer extension analyses of psaA transcripts indicate that the redox regulation occurs at the principal bacterial promoters,suggesting that the plastid encoded RNA polymerase (PEP) is the target for redox signals.Chloroplast protein fractions containing PEP and other DNA-binding proteins were purified from mustard via heparin-Sepharose chromatography.The biochemical prop-erties of these fractions were analyzed with special emphasis on promoter recognition and specificity,phosphorylation state,and kinase activity.The results demonstrate that the LTR involves the action of small DNA-binding proteins; three of them exhibit specific changes in the phosphorylation state.Auto-phosphorylation assays,in addition,exhibit large differ-ences in the activity of endogenous kinase activities.Chloroplast run-on transcription experiments with the kinase inhib-itor H7 and the reductant DTF indicate that phosphorylation events are essential for the mediation of redox signals toward psaA and psbA transcription initiation,but require the synergistic action of a thiol redox signal.The data support the idea that redox signals from the thylakoid membrane are linked to gene expression via phosphorylation events; however,thismediation appears to require a complex network of interacting proteins rather than a simple phosphorelay.