AbstractArabidopsis uracil phosphoribosyltransferase (UPP) has been identified as an essential protein with a previously unknown moonlighting activity. By analysis of UPP amiRNA mutants, we first confirmed a vital function of UPP in chloroplast development and growth which seems unrelated to nucleotide homeostasis since nucleotide levels were not altered in the mutants. Impaired photosynthesis and reductions in carbohydrate contents in these mutants resulted in an inability to induce flavonoid biosynthesis after high light (HL) treatment. Additionally, levels of raffinose, proline and fumarate, acting as osmoprotectants and carbon-stores were reduced as well. Proteomic analysis suggests a role of UPP in chloroplast proteostasis, particularly under HL, and this was confirmed by immunoblots. PetC, the iron-sulfur protein of the Cytb6f complex, was identified as putative UPP target that is massively reduced in UPP knock-down and knock-out plants, resulting in blocked photosynthetic electron transport, seen by increased unregulated energy dissipation and subsequent damage to the D1 protein of PSII. Proteases, including thylakoid-bound FtsHs, were up-regulated together with components of the chloroplast protein import machinery. We hypothesize that the UPP assists in the targeting of PetC and further interacting proteins to the thylakoid and when this mechanism is impaired in corresponding mutants chloroplast proteases are upregulated as a countermeasure.