@article {1743, title = {A tRNA modification balances carbon and nitrogen metabolism by regulating phosphate homeostasis.}, journal = {Elife}, volume = {8}, year = {2019}, month = {2019 Jul 01}, abstract = {

Cells must appropriately sense and integrate multiple metabolic resources to commit to proliferation. Here, we report that cells regulate carbon and nitrogen metabolic homeostasis through tRNA U-thiolation. Despite amino acid sufficiency, tRNA-thiolation deficient cells appear amino acid starved. In these cells, carbon flux towards nucleotide synthesis decreases, and trehalose synthesis increases, resulting in a starvation-like metabolic signature. Thiolation mutants have only minor translation defects. However, in these cells phosphate homeostasis genes are strongly down-regulated, resulting in an effectively phosphate-limited state. Reduced phosphate enforces a metabolic switch, where glucose-6-phosphate is routed towards storage carbohydrates. Notably, trehalose synthesis, which releases phosphate and thereby restores phosphate availability, is central to this metabolic rewiring. Thus, cells use thiolated tRNAs to perceive amino acid sufficiency, balance carbon and amino acid metabolic flux and grow optimally, by controlling phosphate availability. These results further biochemically explain how phosphate availability determines a switch to a {\textquoteright}starvation-state{\textquoteright}.

}, issn = {2050-084X}, doi = {10.7554/eLife.44795}, author = {Gupta, Ritu and Walvekar, Adhish and Liang, Shun and Rashida, Zeenat and Shah, Premal and Laxman, Sunil} } @article {1987, title = {tRNA wobble-uridine modifications as amino acid sensors and regulators of cellular metabolic state.}, journal = {Curr Genet}, year = {2019}, month = {2019 Nov 22}, abstract = {

Cells must appropriately sense available nutrients and accordingly regulate their metabolic outputs, to survive. This mini-review considers the idea that conserved chemical modifications of wobble (U34) position tRNA uridines enable cells to sense nutrients and regulate their metabolic state. tRNA wobble uridines are chemically modified at the 2- and 5- positions, with a thiol (s2), and (commonly) a methoxycarbonylmethyl (mcm5) modification, respectively. These modifications reflect sulfur amino acid (methionine and cysteine) availability. The loss of these modifications has minor translation defects. However, they result in striking phenotypes consistent with an altered metabolic state. Using yeast, we recently discovered that the s2 modification regulates overall carbon and nitrogen metabolism, dependent on methionine availability. The loss of this modification results in rewired carbon (glucose) metabolism. Cells have reduced carbon flux towards the pentose phosphate pathway and instead increased flux towards storage carbohydrates-primarily trehalose, along with reduced nucleotide synthesis, and perceived amino acid starvation signatures. Remarkably, this metabolic rewiring in the s2U mutants is caused by mechanisms leading to intracellular phosphate limitation. Thus this U34 tRNA modification responds to methionine availability and integratively regulates carbon and nitrogen homeostasis, wiring cells to a {\textquoteright}growth{\textquoteright} state. We interpret the importance of U34 modifications in the context of metabolic sensing and anabolism, emphasizing their intimate coupling to methionine metabolism.

}, issn = {1432-0983}, doi = {10.1007/s00294-019-01045-y}, author = {Gupta, Ritu and Laxman, Sunil} } @article {1164, title = {Thiol trapping and metabolic redistribution of sulfur metabolites enable cells to overcome cysteine overload.}, journal = {Microb Cell}, volume = {4}, year = {2017}, month = {2017 Mar 27}, pages = {112-126}, abstract = {

Cysteine is an essential requirement in living organisms. However, due to its reactive thiol side chain, elevated levels of intracellular cysteine can be toxic and therefore need to be rapidly eliminated from the cellular milieu. In mammals and many other organisms, excess cysteine is believed to be primarily eliminated by the cysteine dioxygenase dependent oxidative degradation of cysteine, followed by the removal of the oxidative products. However, other mechanisms of tackling excess cysteine are also likely to exist, but have not thus far been explored. In this study, we use , which naturally lacks a cysteine dioxygenase, to investigate mechanisms for tackling cysteine overload. Overexpressing the high affinity cysteine transporter, , enabled yeast cells to rapidly accumulate high levels of intracellular cysteine. Using targeted metabolite analysis, we observe that cysteine is initially rapidly interconverted to non-reactive cystine . A time course revealed that cells systematically convert excess cysteine to inert thiol forms; initially to cystine, and subsequently to cystathionine, S-Adenosyl-L-homocysteine (SAH) and S-Adenosyl L-methionine (SAM), in addition to eventually accumulating glutathione (GSH) and polyamines. Microarray based gene expression studies revealed the upregulation of arginine/ornithine biosynthesis a few hours after the cysteine overload, and suggest that the non-toxic, non-reactive thiol based metabolic products are eventually utilized for amino acid and polyamine biogenesis, thereby enabling cell growth. Thus, cells can handle potentially toxic amounts of cysteine by a combination of thiol trapping, metabolic redistribution to non-reactive thiols and subsequent consumption for anabolism.

}, issn = {2311-2638}, doi = {10.15698/mic2017.04.567}, author = {Deshpande, Anup Arunrao and Bhatia, Muskan and Laxman, Sunil and Bachhawat, Anand Kumar} }