Role of Mg-chelatase on intracellular signal transduction

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Mg-chelatase, which is composed by CHLI, CHLD, and CHLH subunits, catalyzes the the first committed step of chlorophyll biosynthetic pathway. In plants, it is known that the expression of nuclear-encoded photosynthesis related genes is coupled with chloroplast functionality. In Arabidopsis, mutants termed gun (genome uncoupled) were identified in which intracellular signaling was disrupted. Screening of gun mutants revealed many mutations were originated in mis-sense mutation of CHLH, suggesting Mg-chelatase functions not only for chlorophyll biosynthesis but also for the intracellular chloroplast to nucleus signal transduction (retrograde signaling).In this study, we have constructed reconstitution system of Mg-chelatase activity and revealed that gun phenotype is mainly related to the repression of Mg-chelatase activity. However, it is not clear why such repression caused the accumulation of CHLH protein in vivo. Analysis of functionality and conformation of Mg-chelatase complex will give novel insight into the retrograde signaling pathway in plant cells.