Multiple chloroplast signals are involved in nuclear gene expression

Mauro Ceol, Alexei Shapiguzov, Mounia Heddad and Jean-David Rochaix

Departments of Molecular Biology and Plant Biology, University of Geneva, Switzerland

In Chlamydomonas the chloroplast genome contains more than hundred genes. However the chloroplast contains thousands of different proteins, the vast majority of which are encoded by the nuclear genome. It is well established that signals are exchanged between these two cellular compartments to ensure coordinate expression of many genes. Some nuclear genes encode proteins that are imported in the chloroplast where they are required for chloroplast gene expression. Such a direct effect is not possible for retrograde signaling as no protein export machinery has been discovered in the chloroplast until now. In order to modulate nuclear gene expression the chloroplast releases signals that trigger the induction or the repression of specific sets of genes in the nucleus. The exact nature of these signals is still unclear, but the chloroplast origin of at least four of them is known: the accumulation of chlorophyll precursors, the redox state of the plastoquinone pool, the production of singlet oxygen and the activity of the cytochrome b6f complex (Rodermel S., Trends Plant Sci. 2001, 6: 471-478). The Flu-Like Protein (FLP) is a negative regulator of the chlorophyll biosynthetic pathway and is required in order to avoid cellular damage due to accumulation of chlorophyll precursors (Falciatore et al., Genes&Dev 2005, 19: 176-187). The expression of the FLP gene is regulated by retrograde signals at the level of transcription and splicing. We have used biochemical and genetic tools in order to understand which factors are responsible for the regulation of FLP gene expression.

e-mail address of presenting author: Mauro.Ceol@molbio.unige.ch