CHLOROPHYLL PRECURSORS ARE SIGNALS OF CHLOROPLAST ORIGIN INVOLVED IN LIGHT INDUCTION OF NUCLEAR HEAT-SHOCK GENES

Coordination between the activities of organelles and the nucleus requ ires the exchange of signals, Using Chlamydomonas, we provide evidence that plastid-derived chlorophyll precursors may replace light in the induction of two nuclear heat-shock genes (HSP70A and HSP70B) and thus qualify as plastidic signal. Mutants defective in the synthesis of Mg -protoporphyrin IX were no longer inducible by light. Feeding of Mg-pr otoporphyrin IX or its dimethyl ester to wild-type or mutant cells in the dark resulted in induction. The analysis of HSP70A promoter mutant s that do or do not respond to light revealed that these chlorophyll p recursors specifically activate the light signaling pathway. Activatio n of gene expression was not observed when protoporphyrin IX, protochl orphyllide, or chlorophyllide were added: A specific interaction of de fined chlorophyll precursors with factor(s) that regulate nuclear gene expression is suggested.