Involvement of heme and Mg-tetrapyrroles in retrograde signaling by the chloroplast

Erika D. v. Gromoff1, Ali Alawady2, Linda Meinecke1, Bernhard Grimm2, and Christoph F. Beck1

1Fakultät für Biologie, Institut für Biologie III, Universität Freiburg, Schaenzlestrasse 1, D-79104 Freiburg, Germany 2 Institut fŸr Biologie/Pflanzenphysiologie, Humboldt Universität, Philippstrasse 13, D-10115 Berlin, Germany.

The involvement of chlorophyll precursors like Mg-protoporphyrin IX (MgProto) in signaling from chloroplast to nucleus is supported by experiments performed with Chlamydomonas and higher plants. In Chlamydomonas, MgProto has been implicated in the light induction of a subset of nuclear genes, the best studied being HSP70A. Mutants affected in the heterotrimeric enzyme Mg-chelatase and thus unable to synthesize chlorophyll provided new insight into the signaling role of tetrapyrroles. In mutants defective in Mg-chelatase subunits D or H and thus impaired in MgProto synthesis, light induction of HSP70A was observed although MgProto has previously been assigned an important role in this induction. Inspection of tetrapyrrole steady state levels in the mutants revealed a reduction in Mg-tetrapyrroles but an increase in soluble heme levels. Also, in contrast to the situation in wild type, heme levels in the mutants remained high during shift from dark to light. This suggested a possible regulatory role for heme which in Chlamydomonas appears to be produced only in plastids. Indeed, feeding of hemin to cultures in the dark induced HSP70A. This induction requires the same enhancer (called PRE) that previously has been shown to mediate the induction by MgProto. Other nuclear genes that harbour a PRE in their promoters were also induced by hemin feeding. We propose that both, MgProto and heme, may serve as retrograde plastid signals that regulate a subset of nuclear genes.

e-mail address of presenting author: beck@uni-freiburg.de