Chianelli M S, Stella C A, Sáenz D A, Ramos E H, Kotliar N, Mattoon J R
Departmento de Bioquímica, Facultad de Medicina, Universidad de Buenos Aires-CONICET, Argentina.
Cell Mol Biol (Noisy-le-grand). 1996 Sep;42(6):847-57.
A yeast mutant defective in permeases S1 and S2 which transport L-leucine was isolated from a parental strain already deficient in the general amino acid permease, GAP1. The mutant was selected as a spontaneous, trifluoroleucine-resistant (TFLR) strain. Full resistance depended upon the presence of two unlinked mutant genes designated let1 and let2. The let1 mutation completely inactivates the high-affinity leucine transport system defined kinetically as S1. Although the let2 mutation caused a marked decrease in the Jmax of the low-affinity transport system, S2, residual leucine transport in the let1 let2 gap1 mutant had the same KT as in the LET1 LET2 gap1 parent. The mutant exhibited a marked decrease in growth on minimal medium containing leucine, isoleucine or valine as a sole nitrogen source. Moreover, assimilation of methionine, phenylalanine, serine and threonine was decreased, whereas basic and acidic amino acids supported normal growth. This indicates that at least one of the leucine permeases has a fairly broad, but still limited, specificity. Reversion of the gap1 gene restored leucine transport. The revertant was sensitive to TFL when grown on proline but resistant when NH4+ was the nitrogen source. The previously published mutations (shr3, aat1, lup1 or raa) would not be related to either LET1 or LET2.
从已经缺乏通用氨基酸通透酶GAP1的亲本菌株中分离出一种在转运L-亮氨酸的通透酶S1和S2方面存在缺陷的酵母突变体。该突变体是作为一种自发的三氟亮氨酸抗性（TFLR）菌株筛选出来的。完全抗性取决于两个不连锁的突变基因let1和let2的存在。let1突变完全使动力学上定义为S1的高亲和力亮氨酸转运系统失活。尽管let2突变导致低亲和力转运系统S2的Jmax显著降低，但let1 let2 gap1突变体中的残余亮氨酸转运与LET1 LET2 gap1亲本中的KT相同。该突变体在以亮氨酸、异亮氨酸或缬氨酸作为唯一氮源的基本培养基上生长时表现出显著下降。此外，甲硫氨酸、苯丙氨酸、丝氨酸和苏氨酸的同化作用降低，而碱性和酸性氨基酸支持正常生长。这表明至少一种亮氨酸通透酶具有相当广泛但仍有限的特异性。gap1基因的回复突变恢复了亮氨酸转运。回复突变体在脯氨酸上生长时对TFL敏感，但以NH4+作为氮源时具有抗性。先前发表的突变（shr3、aat1、lup1或raa）与LET1或LET2均无关。
