Schachner E, Aschhoff H J, Kersten H
Eur J Biochem. 1984 Mar 15;139(3):481-7. doi: 10.1111/j.1432-1033.1984.tb08031.x.
Higher eukaryotes contain tRNA transglycosylases that incorporate the guanine derivative queuine from the nutritional environment into specific tRNAs by exchange with guanine at position 34. Alterations in the queuosine content of specific tRNAs are suggested to be involved in regulatory mechanisms of major routes of metabolism during differentiation. Dictyostelium discoideum has been applied as a model to investigate the function of queuine or queuine-containing tRNAs. Axenic strains are supplied with queuine by peptone, but they grow equally well in a defined queuine-free medium. Queuine-lacking amoebae, starved in suspension culture for 24 h, lose their ability to differentiate into stalk cells and spores, whereas amoebae sufficiently supplied with queuine will overcome this metabolic stress and undergo further development when plated on agar. The results presented here show that D(-)-lactate occurs in the slime mould in millimolar amounts and that its level is remarkably decreased in queuine-lacking cells after 24 h of starvation in suspension culture. On isoelectric-focusing polyacrylamide gels, nine different forms of NAD-dependent D(-)-lactate dehydrogenase can be separated from extracts of vegetative cells, and six forms from extracts of the starved cells. Under queuine limitation, one form is missing in the starved cells. Low amounts of L(+)-lactate are usually found in vegetative amoebae but significantly less in queuine-lacking cells. Five forms of NAD-dependent L(+)-lactate dehydrogenase are detectable in extracts from vegetative, queuine-treated cells, and slight alterations occur in queuine-deficient amoebae. In the starved cells only one form of L(+)-lactate dehydrogenase is found, irrespective of the supply of queuine to the cells. A cytochrome of type b with an absorption maximum at 559 nm accumulates during starvation only in queuine-lacking cells; it might be a component of an NAD-independent lactic acid oxidoreductase as is cytochrome b 557 in yeast and be responsible for the reduced level of lactate in cells lacking queuine in tRNA.
高等真核生物含有tRNA转糖基酶，该酶可通过与34位的鸟嘌呤交换，将营养环境中的鸟嘌呤衍生物queuine掺入特定的tRNA中。特定tRNA中queuosine含量的变化被认为参与了分化过程中主要代谢途径的调控机制。盘基网柄菌已被用作研究queuine或含queuine的tRNA功能的模型。无菌株系通过蛋白胨获得queuine，但它们在不含queuine的限定培养基中生长同样良好。缺乏queuine的变形虫在悬浮培养中饥饿24小时后，失去分化为柄细胞和孢子的能力，而充分供应queuine的变形虫在接种到琼脂上时将克服这种代谢应激并进一步发育。此处呈现的结果表明，D(-)-乳酸在黏菌中以毫摩尔量存在，并且在悬浮培养中饥饿24小时后，缺乏queuine的细胞中其水平显著降低。在等电聚焦聚丙烯酰胺凝胶上，从营养细胞提取物中可分离出九种不同形式的NAD依赖性D(-)-乳酸脱氢酶，从饥饿细胞提取物中可分离出六种形式。在queuine受限的情况下，饥饿细胞中缺少一种形式。营养性变形虫中通常发现少量L(+)-乳酸，但在缺乏queuine的细胞中显著减少。在营养性、经queuine处理的细胞提取物中可检测到五种形式的NAD依赖性L(+)-乳酸脱氢酶，在缺乏queuine的变形虫中发生轻微变化。在饥饿细胞中，无论细胞是否供应queuine，仅发现一种形式的L(+)-乳酸脱氢酶。一种在559nm处具有最大吸收峰的b型细胞色素仅在缺乏queuine的饥饿细胞中积累；它可能是一种不依赖NAD的乳酸氧化还原酶的成分，就像酵母中的细胞色素b 557一样，并负责tRNA中缺乏queuine的细胞中乳酸水平的降低。
