Magill J M, Magill C W
J Bacteriol. 1975 Oct;124(1):149-54. doi: 10.1128/jb.124.1.149-154.1975.
Observations presented in this paper point to the presence of dual transport mechanisms for the base adenine in Neurospora crassa. Competition for transport, as well as growth inhibition studies using an ad-1 auxotroph, show that the purine bases adenine, guanine, and hypoxanthine share at least one transport mechanism which is insensitive to adenosine, cytosine, and a variety of other purine base analogues. On the other hand, uptake of adenine by an ad-8 mutant strain unable to transport [8-14C]hypoxanthine at any concentration was not inhibited by guanine or hypoxanthine. This observation demonstrates the existence of an adenine-specific transport system which was also found to be insensitive to inhibition by other purine base analogues, adenosine or cytosine. Recombination analysis of ad-8 by wild-type crosses showed that the inability to transport [8-14C]hypoxanthine was a consequence of the ad-8 lesion or a closely linked mutation. Saturation plots of each system gave intermediary plateaus and nonlinear reciprocal plots which, based on comparison with pure enzyme kinetic analysis, suggest that either each system consists of two or more uptake systems, at least one of which exhibits cooperativity, or that each system is a single uptake mechanism which possesses more than two binding sites where the relative affinity for the purine base first decreases and then increases as the sites are filled.
本文所呈现的观察结果表明，粗糙脉孢菌中存在腺嘌呤的双重转运机制。转运竞争以及使用ad - 1营养缺陷型进行的生长抑制研究表明，嘌呤碱基腺嘌呤、鸟嘌呤和次黄嘌呤共享至少一种对腺苷、胞嘧啶以及多种其他嘌呤碱基类似物不敏感的转运机制。另一方面，任何浓度下都无法转运[8 - 14C]次黄嘌呤的ad - 8突变菌株对腺嘌呤的摄取不受鸟嘌呤或次黄嘌呤的抑制。这一观察结果证明了存在一种腺嘌呤特异性转运系统，该系统也被发现对其他嘌呤碱基类似物、腺苷或胞嘧啶的抑制不敏感。通过野生型杂交对ad - 8进行的重组分析表明，无法转运[8 - 14C]次黄嘌呤是ad - 8损伤或紧密连锁突变的结果。每个系统的饱和图给出了中间平台和非线性倒数图，基于与纯酶动力学分析的比较，这表明要么每个系统由两个或更多摄取系统组成，其中至少一个表现出协同性，要么每个系统是一种单一摄取机制，它拥有两个以上的结合位点，随着位点被占据，对嘌呤碱基的相对亲和力先降低后增加。
