Pyrimidine nucleotide synthesis is initiated in chloroplasts by Aspartate transcarbamoylase (ATCase) converting Aspartate (Asp) and Carbamoylphosphate to Carbamoylaspartate (CarAsp) which is exported to the cytosol to supply UTP synthesis. ATCase represents the rate limiting step in this pathway. So far, the only known pathway to supply the chloroplast with UTP is composed of the Uracil (Ura) importer PLUTO and Uracil phosphoribosyltransferase (UPP) converting uracil and phosphoribosylpyrophosphate to UMP. Further steps in this salvage pathway then produce UTP. Besides acting directly in the salvage pathway, UPP exhibits moonlighting activity as essential component in the establishment of photosynthesis. Preliminary data point to a contribution of ATCase and UPP in acclimation to cold temperatures.

Figure: Nucleotide and energy metabolism contribute to plant acclimation. Created with BioRender.com

Ohler L, Niopek-Witz S, Mainguet SM, Möhlmann T (2019) Physiological function of pyrimidine salvage and interaction with chloroplast biogenesis. Plant Physiol. 180, 1816-1828 doi: 10.1104/pp.19.00329

Bellin L, del Caño-Ochoa F, Velázquez-Campoy A, Möhlmann T, Ramón-Maiques S (2020) UMP inhibition and sequential firing in aspartate transcarbamoylase open ways to regulate plant growth. bioRxiv doi:10.1101/2020.07.17.20823