A tRNA modification balances carbon and nitrogen metabolism by regulating phosphate homeostasis, to couple metabolism to cell cycle progression.

Cells must appropriately sense and integrate multiple metabolic resources to commit to proliferation. Here, we report that cells regulate nitrogen (amino acid) and carbon metabolic homeostasis through tRNA U34-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 metabolic a starvation-signature. Thiolation mutants have only minor translation defects. However, these cells exhibit strongly decreased expression of phosphate homeostasis genes, mimicking a 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, and balance amino acid and carbon metabolic flux to maintain metabolic homeostasis, by controlling phosphate availability. These results further biochemical explain how phosphate availability determines a switch to a ‘starvation-state’.细胞必须精准感知并整合多种代谢资源，方可启动增殖程序。本研究发现，细胞通过tRNA U34硫修饰（tRNA U34-thiolation）调控氮（氨基酸）与碳代谢稳态。即便氨基酸供应充足，tRNA硫修饰缺陷的细胞仍会呈现氨基酸饥饿的表型。此类细胞中，流向核苷酸合成的碳通量降低，而海藻糖合成增强，最终产生代谢饥饿特征。硫修饰缺陷突变体仅存在轻微的翻译缺陷，但这类细胞的磷稳态基因表达水平显著下调，模拟出磷限制的细胞状态。磷水平降低会触发代谢重编程，使葡萄糖-6-磷酸流向储存性碳水化合物的合成途径。值得注意的是，海藻糖合成可释放磷并恢复磷的有效利用，这是此次代谢重编程的核心环节。综上，细胞通过硫修饰tRNA感知氨基酸供应水平，并通过调控磷的有效利用平衡氨基酸与碳代谢通量，从而维持代谢稳态。本研究结果进一步从生物化学层面阐明了磷的有效利用水平如何决定细胞向“饥饿状态”的转换。