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PAP8, a double agent of greening in Angiosperms

The initial greening of the angiosperms involves a light activation of the photoreceptors that trigger photomorphogenesis, followed by the development of chloroplasts. In these organelles, the construction of the photosynthetic apparatus depends on the coordination of the expression of nuclear and plastid genes. Researchers at our laboratory are studying the expression of PAP8, an essential subunit of the RNA polymerase encoded by the plastid, localized and active in both the plastids and the nucleus.

Published on 30 November 2020
During the early stages of their development, flowering plants, or Angiosperms, turn green only in the presence of light following a program called photomorphogenesis. The cells of the leaf then accumulate all the proteins necessary for photosynthesis during the formation of mature chloroplasts, which are characterized by their green color. This development of the functional chloroplast requires the orchestrated expression of genes encoded in the nuclear DNA as well as in the chloroplast DNA. Many mysteries remain vivid around the molecular and genetic mechanisms that allow the coordinated expression of these two genomes. These mechanisms however are essential for the optimal use of the resources stored in the seed while the young seedling searching for light is more or less deeply buried in the soil.

In most Angiosperms, the chloroplast PEP RNA polymerase is a protein complex whose catalytic core is encoded by the plastid itself. The PEP associates with 12 proteins encoded by nuclear genes called PAP (PEP-Associated Proteins) whose expression is regulated by light. In the presence of these PAPs, the photosynthesis-associated plastid genes are activated. PAP8 is an essential subunit of the RNA polymerase because loss of its function leads to plastids that fail to develop, resulting in a complete loss of green pigmentation within a lethal albinism syndrome (Photograph below).
When the PAP8 gene is not expressed, the seedlings have an albino phenotype (left seedling).

In this study, researchers at our laboratory examined the regulation and function of the PAP8 gene in vivo. They showed that the PAP8 protein, through unknown mechanisms, is localized in the nucleus and in the chloroplast with a distinct role in the two compartments. Its absence disrupts the nuclear signalling pathway of the PhyB photoreceptor. The seedling is less reactive to light, the photomorphogenesis program is suspended while the antagonistic dark development program is maintained. Based on the PAP8 signaling sequences, the researchers generated artificial proteins, variants "mono-localized" either in the nucleus or in the chloroplast, showing that the functions of PAP8 cannot be uncoupled (no variant protein can completely correct the defects of the mutant). While in the chloroplast, PAP8 is associated with PEP and remains essential for greening, its nuclear function seems to be strongly involved in the mere coordination of nuclear genes expression with that of chloroplast genes.
The data obtained indicate that a retrograde movement (with reference to the signals sent by the organelles towards the nucleus) of the PAP8 protein towards the nucleus could, together with other proteins of the complex, regulate the expression of nuclear genes depending on the state of the plastids and their ability to produce the proteins of the photosynthetic apparatus, a mechanism described since the end of the 1990s under the term genetic coupling. Because of its dual localization, PAP8 represents a new regulatory component that links photomorphogenesis and biogenesis of chloroplasts. PAP8 is thus a new member of the family of nucleo-plastidic proteins involved in chloroplast biogenesis.

Artwork by Robert Blanvillain illustrating some key results related to the localization of PAP8, the phenotypic analysis of the mutant, the promoter analysis as well as the structural analysis of PAP8 and its interaction with HMR/PAP5.

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