Molecular and cellular research on aromatic amino acids production, by microorganisms.

在大肠杆菌中，苯丙氨酸的主动转运被认为是由两个不同的系统进行的，AroP和PheP。在芳香族氨基酸转运蛋白缺陷的E.大肠杆菌菌株（ΔaroP ΔpheP Δmtr Δtna ΔtyrP）。该菌株对苯丙氨酸的摄取在支链氨基酸存在下被显著抑制。遗传分析和转运研究表明，LIV-I/LS系统是一种支链氨基酸转运蛋白，由两种周质结合蛋白（LIV结合蛋白（LIV-I系统）和LS结合蛋白（LS系统））和膜组分LivHMGF组成，参与苯丙氨酸积累。E.杆菌细胞在-LIV-I和LS系统中，苯丙氨酸的Km值分别为19和30 μM。在LIV-I系统中观察到异亮氨酸、亮氨酸和缬氨酸对苯丙氨酸摄取的竞争性抑制，令人惊讶的是，在LS系统中也观察到这种抑制，根据与纯化的LS结合蛋白的结合研究结果，LS系统被认为是亮氨酸特异性的。我们发现LS系统能够以与远亮氨酸相当的亲和力转运异亮氨酸和缬氨酸，并且LIV-I系统能够以低于用其它底物所见的亲和力转运酪氨酸。LIV-I/LS系统对苯丙氨酸积累的生理意义在苯丙氨酸营养缺陷型E.大肠杆菌菌株在各种条件下。

In Escherichia coli, the active transport of phenylalanine is considered to be performed by two different systems, AroP and PheP However, a low. level of accumulation of phenylalanine was observed in an aromatic amino acid transporter-deficient E. coli strain (ΔaroP ΔpheP Δmtr Δtna ΔtyrP). The uptake of phenylalanine by this strain was significantly inhibited in the presence of branched-chain amino acids. Genetic analysis and transport studies revealed that the LIV-I/LS system, which is a branched-chain amino acid transporter consisting of two periplasmic binding proteins, the LIV-binding protein (LIV-I system) and LS-binding protein (LS system), and membrane components, LivHMGF, is involved in phenylalanine accumulation.in. E. coli. cells. The Km values for phenylalanine in the-LIV-I and LS systems were determined to be 19 and 30 μM, respectively. Competitive inhibition of phenylalanine uptake by isoleucine, leucine, and valine was observed for the LIV-I system and, surprisingly, also for the LS system, which has been assumed to be leucine specific on the basis of the results of binding studies with the purified LS-binding protein. We found that the LS system is capable of transporting isoleucine and valine with affinity comparable to that far leucine and that the LIV-I system is able to transport tyrosine with affinity lower than that seen with other substrates. The physiological importance of the LIV-I/LS system for phenylalanine accumulation was revealed in the growth of phenylalanine-auxotrophic E. coli strains under various conditions.