National Institute of Plant Genome Research, New Delhi, India.
Fungal Genet Biol. 2013 May;54:15-24. doi: 10.1016/j.fgb.2013.02.006. Epub 2013 Feb 27.
N-Acetylglucosamine (GlcNAc) is an important signaling molecule that plays multiple roles in Candida albicans. Induction of galactose metabolic pathway by GlcNAc is an intriguing aspect of C. albicans biology. In order to investigate the role of galactose metabolic genes (GAL genes) in presence of GlcNAc, we created knockouts of galactokinase (GAL1) and UDP galactose epimerase (GAL10) genes. These mutants failed to grow on galactose and also showed lower growth rate in presence of GlcNAc. Interestingly, expression of GAL genes in presence of GlcNAc was higher in gal1Δ strain relative to that of wild type strain. Moreover, no GlcNAc induced upregulation of GAL genes was observed in the gal10Δ strain suggesting that UDP galactose epimerase is essential for GlcNAc induced activation of GAL genes. GlcNAc induced expression of GAL genes was also investigated in GlcNAc metabolic pathway triple mutant N216 (hxk1Δ nag1Δ dac1Δ). Interestingly, in this mutant the GAL genes are neither induced nor repressed and remain derepressed as found on a neutral carbon source such as glycerol, suggesting that catabolism of GlcNAc play an important role in the expression of GAL genes. GC/MS analysis of derivatized metabolites revealed a significant accumulation of galactose in the gal1Δ strain while no galactose was detected in gal10Δ and N216 strain. Solution-state NMR spectroscopy using N-acetyl-¹³C₁-glucosamine confirmed the flow of ¹³C label from GlcNAc to galactose. Thus, internal galactose synthesized via UDP galactose pathway from GlcNAc metabolites acts as the inducer of GAL genes in presence of GlcNAc.
N-乙酰葡萄糖胺（GlcNAc）是一种重要的信号分子，在白色念珠菌中发挥多种作用。GlcNAc 诱导半乳糖代谢途径是白色念珠菌生物学的一个有趣方面。为了研究半乳糖代谢基因（GAL 基因）在 GlcNAc 存在时的作用，我们敲除了半乳糖激酶（GAL1）和 UDP 半乳糖差向异构酶（GAL10）基因。这些突变体不能在半乳糖上生长，并且在 GlcNAc 存在时生长速度也较慢。有趣的是，与野生型菌株相比，GAL 基因在 GlcNAc 存在时的表达在 gal1Δ 菌株中更高。此外，在 gal10Δ 菌株中没有观察到 GlcNAc 诱导的 GAL 基因上调，表明 UDP 半乳糖差向异构酶对于 GlcNAc 诱导的 GAL 基因激活是必不可少的。还研究了 GlcNAc 代谢途径三突变体 N216（hxk1Δ nag1Δ dac1Δ）中 GlcNAc 诱导的 GAL 基因表达。有趣的是，在这个突变体中，GAL 基因既没有被诱导也没有被抑制，并且仍然像在中性碳源如甘油上一样被去阻遏，这表明 GlcNAc 的分解代谢在 GAL 基因的表达中起着重要作用。衍生代谢物的 GC/MS 分析显示 gal1Δ 菌株中半乳糖显著积累，而 gal10Δ 和 N216 菌株中未检测到半乳糖。使用 N-乙酰-¹³C₁-葡萄糖胺的溶液态 NMR 光谱学证实了¹³C 标记从 GlcNAc 到半乳糖的流动。因此，通过 GlcNAc 代谢物的 UDP 半乳糖途径合成的内源性半乳糖在 GlcNAc 存在时作为 GAL 基因的诱导物。
