Pyrimidine Salvage: Physiological Functions and Interaction with Chloroplast Biogenesis

Abstract

The synthesis of pyrimidine nucleotides, an essential process in every organism, is accomplished by de novo synthesis or by salvaging pyrimdines from e.g. nucleic acid turnover. Here, we identify two Arabidopsis (Arabidopsis thaliana) uridine/cytidine kinases, UCK1 and UCK2, which are located in the cytosol and are responsible for the majority of pyrimidine salvage activity in vivo. In addition, the chloroplast has an active uracil salvage pathway. Uracil phosphoribosyltransferase (UPP) catalyzes the initial step in this pathway and is required for the establishment of photosynthesis, as revealed by analysis of upp mutants. The upp knockout mutants are unable to grow photoautotrophically, and knockdown mutants exhibit a variegated phenotype, with leaves that have chlorotic pale areas. Moreover, the upp mutants did not show altered expression of chloroplast-encoded genes, but transcript accumulation of the LIGHT HARVESTING COMPLEX B nuclear genes LHCB1.2 and LHCB2.3 was markedly reduced. An active UPP homolog from Escherichia coli failed to complement the upp mutant phenotype when targeted to the chloroplast, suggesting that the catalytic function of UPP is not the important factor for the chloroplast phenotype. Indeed, the expression of catalytically inactive Arabidopsis UPP, generated by introduction of point mutations, did complement the upp chloroplast phenotype. These results suggest that UPP has a vital function in chloroplast biogenesis unrelated to its catalytic activity and driven by a moonlighting function.

CTP Synthase 2 From Arabidopsis thaliana Is Required for Complete Embryo Development

“…In contrast, differentiated cells maintain their nucleotide pools via salvage and compensate catabolic processes by minor de novo synthesis activity (Zrenner et al, 2006). Consequently, the external-supplied nucleoside cytidine, which can be imported in plants by ENT3 (Traub et al, 2007) and can be phosphorylated to CMP by uridine/cytidine kinase (Ohler et al, 2019), should allow for a compensation of lost CTP synthase activity in ctps2 seedlings. However, the application of 1 mM cytidine did not overcome the germination phenotype of ctps2 seeds (Figure 1).…”

Acclimation in plants – the Green Hub consortium

“…(3) Ascorbate and thiamine are involved in the control of photosynthetic efficiency and coordinate acclimation‐associated responses between chloroplasts and mitochondria (de Souza Chaves et al ., 2019; Rosado‐Souza et al ., 2019; Feitosa‐Araujo et al ., 2020; Obata et al ., 2020; Rosado‐Souza et al ., 2020). (4) Enzymes involved in nucleotide biosynthesis/salvage perform moonlighting functions and contribute to the coordination of nuclear and organellar gene expression (Möhlmann et al ., 1994; Ohler et al ., 2019; Schmid et al ., 2019). (5) Cold‐induced modulation of stromal sugar levels contributes to freezing tolerance (Nägele and Heyer, 2013; Weiszmann et al ., 2018; Patzke et al ., 2019).…”

“…This interplay is triggered by the stromal proteins uracil phosphoribosyl transferase (UPP) and (plastidic) UMP kinase (PUMPKIN) since both enzymes exhibit moonlighting functions. For example, apart from its catalytic activity, UPP is involved in chloroplast biogenesis and affects the expression of photosynthesis‐associated nuclear genes (Ohler et al ., 2019), while PUMPKIN associates with plastid transcripts and its loss‐of‐function mutants exhibit altered PGE (Schmid et al ., 2019) (process 4 in Figure 2). These findings represent the starting point for further analysis of the interaction between PGE and nucleotide metabolism.…”

Nucleotide Metabolism in Plants

“…6, no. 35;Ohler et al, 2019) to avoid a futile cycle of pyrimidine nucleotide dephosphorylation and pyrimidine nucleoside salvage. Cytidine is deaminated to uridine by a cytosolic cytidine deaminase (CDA; Fig.…”