SUMMARYUridine nucleotides can be formed by energy-consuming de novo synthesis or by the energy-saving recycling of nucleobases resulting from nucleotide catabolism. Uracil phosphoribosyltransferases (UPRTs; EC 2.4.2.9) are involved in the salvage of pyrimidines by catalyzing the formation of uridine monophosphate (UMP) from uracil and phosphoribosylpyrophosphate. To date, UPRTs are described as non-essential, energy-saving enzymes. In the present work, the six genes annotated as UPRTs in the Arabidopsis genome are examined through phylogenetic and functional complementation approaches and the available T-DNA insertion mutants are characterized. We show that a single nuclear gene encoding a protein targeted to plastids, UPP, is responsible for almost all UPRT activity in Arabidopsis. The inability to salvage uracil caused a lightdependent dramatic pale-green to albino phenotype, dwarfism and the inability to produce viable progeny in loss-of-function mutants. Plastid biogenesis and starch accumulation were affected in all analysed tissues, with the exception of stomata. Therefore we propose that uracil salvage is of major importance for plant development.
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.
Production of renewable fuels and chemicals is an absolute requirement for the sustainability of societies. This fact has been neglected during the past century as cheap and abundant, yet not renewable, sources of hydrocarbons were available. Since fossil fuel availability is decreasing, biological production of fuels and chemicals has been proposed to be a potential alternative to fossil sources. Higher alcohols (from C₃ to C₅) are useful substitutes for gasoline because of their high energy density and low hygroscopicity and are important feedstocks for other chemicals. Some Clostridia species are known to naturally ferment sugars to isopropanol and 1-butanol. However, other C₃ to C₅ alcohols are not produced in large quantities by natural microorganisms. A non-fermentative strategy to produce a broad range of higher alcohols has been devised using the ubiquitous keto acid biosynthetic pathways. This review provides a current overview of these different strategies.
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