Magill C W, Sabina R L, Garber T L, Magill J M
J Bacteriol. 1982 Mar;149(3):941-7. doi: 10.1128/jb.149.3.941-947.1982.
Guanine is transported into germinated conidia of Neurospora crassa by the general purine base transport system. Guanine uptake is inhibited by adenine and hypoxanthine but not xanthine. Guanine phosphoribosyltransferase (GPRTase) activity was demonstrated in cell extracts of wild-type germinated conidia. The Km for guanine ranged from 29 to 69 micro M in GPRTase assays; the Ki for hypoxanthine was between 50 and 75 micro M. The kinetics of guanine transport differ considerably from the kinetics of GPRTase, strongly suggesting that the rate-limiting step in guanine accumulation in conidia is not that catalyzed by GPRTase. Efflux of guanine or its metabolites appears to have little importance in the regulation of pools of guanine or guanine nucleotides since very small amounts of 14C label were excreted from wild-type conidia preloaded with [8-14C]guanine. In contrast, excretion of purine bases, hypoxanthine, xanthine, and uric acid appears to be a mechanism for regulation of adenine nucleotide pools (Sabina et al., Mol. Gen. Genet. 173:31-38, 1979). No label from exogenous [8-14C]guanine was ever found in any adenine nucleotides, nucleosides, or the base, adenine, upon high-performance liquid chromatography analysis of acid extracts from germinated conidia of wild-type of xdh-l strains. The 14C label from exogenous [8-14C]guanine was found in GMP, GDP, GTP, and the GDP sugars as well as in XMP. Xanthine and uric acid were also labeled in wild-type extracts. Similar results were obtained with xdh-l extracts except that uric acid was not present. The labeled xanthine and XMP strongly suggest the presence of guanase and xanthine phosphoribosyltransferase in germinated conidia.
鸟嘌呤通过一般嘌呤碱转运系统被转运到粗糙脉孢菌的萌发分生孢子中。腺嘌呤和次黄嘌呤可抑制鸟嘌呤的摄取，但黄嘌呤不会。在野生型萌发分生孢子的细胞提取物中证实了鸟嘌呤磷酸核糖转移酶（GPRTase）的活性。在GPRTase测定中，鸟嘌呤的Km值范围为29至69微摩尔；次黄嘌呤的Ki值在50至75微摩尔之间。鸟嘌呤转运的动力学与GPRTase的动力学有很大差异，这强烈表明分生孢子中鸟嘌呤积累的限速步骤不是由GPRTase催化的步骤。鸟嘌呤或其代谢产物的外流在鸟嘌呤或鸟嘌呤核苷酸池的调节中似乎不太重要，因为从预先加载了[8-14C]鸟嘌呤的野生型分生孢子中排出的14C标记物非常少。相比之下，嘌呤碱、次黄嘌呤、黄嘌呤和尿酸的排泄似乎是调节腺嘌呤核苷酸池的一种机制（萨比娜等人，《分子遗传学与普通遗传学》173:31-38，1979）。在对野生型xdh-l菌株萌发分生孢子的酸提取物进行高效液相色谱分析时，在任何腺嘌呤核苷酸、核苷或碱基腺嘌呤中都未发现来自外源[8-14C]鸟嘌呤的标记。来自外源[8-14C]鸟嘌呤的14C标记物在GMP、GDP、GTP、GDP糖以及XMP中被发现。野生型提取物中的黄嘌呤和尿酸也被标记。除了没有尿酸外，xdh-l提取物也得到了类似的结果。标记的黄嘌呤和XMP强烈表明在萌发分生孢子中存在鸟嘌呤酶和黄嘌呤磷酸核糖转移酶。
