Glycosyltransferase Slr1064 regulates carbon metabolism by modulating the levels of UDP-GlcNAc in Synechocystis sp. PCC 6803.

Glycosyltransferases (GTs) are enzymes that transfer sugars to various targets. They play important roles in diverse biological processes, including photosynthesis, cell motility, exopolysaccharide biosynthesis, and lipid metabolism; however, their involvement in regulating carbon metabolism in Synechocystis sp. PCC 6803 has not been reported. We identified a novel GT protein, Slr1064, involved in carbon metabolism. The effect of slr1064 deletion on the growth of Synechocystis cells and functional mechanisms of Slr1064 on carbon metabolism were thoroughly investigated through physiological, biochemistry, proteomic, and metabolic analyses. We found that this GT, which is mainly distributed in the membrane compartment, is essential for the growth of Synechocystis under heterotrophic and mixotrophic conditions, but not under autotrophic conditions. The deletion of slr1064 hampers the turnover rate of Gap2 under mixotrophic conditions and disrupts the assembly of the PRK/GAPDH/CP12 complex under dark culture conditions. Additionally, UDP-GlcNAc, the pivotal metabolite responsible for the O-GlcNAc modification of GAPDH, is downregulated in the Δslr1064. Our work provides new insights into the role of GTs in carbon metabolism in Synechocystis and elucidate the mechanism by which carbon metabolism is regulated in this important model organism.

Identification and in vitro characterization of UDP-GlcNAc-RNA cap-modifying and decapping enzymes.

In recent years, several noncanonical RNA caps derived from cofactors and metabolites have been identified. Purine-containing RNA caps have been extensively studied, with multiple decapping enzymes identified and efficient capture and sequencing protocols developed for nicotinamide adenine dinucleotide (NAD)-RNA, which allowed for a stepwise elucidation of capping functions. Despite being identified as an abundant noncanonical RNA-cap, UDP-sugar-capped RNA remains poorly understood, which is partly due to its complex in vitro preparation. Here, we describe a scalable synthesis of sugar-capped uridine-guanosine dinucleotides from readily available protected building blocks and their enzymatic conversion into several cell wall precursor-capped dinucleotides. We employed these capped dinucleotides in T7 RNA polymerase-catalyzed in vitro transcription reactions to efficiently generate RNAs capped with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), its N-azidoacetyl derivative UDP-GlcNAz, and various cell wall precursors. We furthermore identified four enzymes capable of processing UDP-GlcNAc-capped RNA in vitro: MurA, MurB and MurC from Escherichia coli can sequentially modify the sugar-cap structure and were used to introduce a bioorthogonal, clickable moiety, and the human Nudix hydrolase Nudt5 was shown to efficiently decap UDP-GlcNAc-RNA. Our findings underscore the importance of efficient synthetic methods for capped model RNAs. Additionally, we provide useful enzymatic tools that could be utilized in the development and application of UDP-GlcNAc capture and sequencing protocols. Such protocols are essential for deepening our understanding of the widespread yet enigmatic GlcNAc modification of RNA and its physiological significance.

Ngk1 kinase-mediated N-acetylglucosamine metabolism promotes UDP-GlcNAc biosynthesis in Saccharomyces cerevisiae.

N-acetylglucosamine (GlcNAc) is an important structural component of the cell wall chitin, N-glycans, glycolipids, and GPI-anchors in eukaryotes. GlcNAc kinase phosphorylates GlcNAc into GlcNAc-6-phosphate, a precursor of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that serves as a substrate for glycan synthesis. Although GlcNAc kinase is found widely in organisms ranging from microorganisms to mammals, it has never been found in the model yeast Saccharomyces cerevisiae. Here, we demonstrate the presence of GlcNAc metabolism for UDP-GlcNAc biosynthesis in S. cerevisiae through Ngk1, a GlcNAc kinase we discovered previously. The overexpression or deletion of Ngk1 in the presence of GlcNAc affected the amount of both UDP-GlcNAc and chitin, suggesting that GlcNAc metabolism via Ngk1 promotes UDP-GlcNAc synthesis. Our data suggest that the Ngk1-mediated GlcNAc metabolism compensates for the hexosamine pathway, a known pathway for UDP-GlcNAc synthesis.

Selective analysis of intracellular UDP-GlcNAc and UDP-GalNAc by hydrophilic interaction liquid chromatography-mass spectrometry.

Uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) is one of the major nucleotide sugars in living organisms and serves as the key donor substrate for the post-translational modification of protein O-GlcNAcylation. It undergoes interconversion to its epimer uridine diphosphate-N-acetylgalactosamine (UDP-GalNAc), which acts as a sugar donor initiating mucin-type O-linked glycosylation. The intracellular levels of the two differ between the cell lines and largely fluctuate in response to metabolic perturbations, and recent studies have focused on the details of their biosynthesis or turnover. However, due to their similar chemical properties, sufficient resolution for the two epimers required non-volatile mobile phases that cannot be applied directly to a mass spectrometer. In this study, to implement simple liquid chromatography-mass spectrometry for UDP-GlcNAc and UDP-GalNAc, we optimized a condition of hydrophilic interaction liquid chromatography-mass spectrometry. We found that the use of ammonium hydroxide and an amide column with an optimized water-acetonitrile ratio, flow rate, and column temperature, provided complete separation of the two. The method allowed the analysis of intracellular levels, a stable isotope-labeled target, and patterns of product ion spectra in a single run with fewer sample preparation steps. The new method can be widely used for mass spectrometric analysis of UDP-GlcNAc and UDP-GalNAc.