Therlt11 andraec1 mutants ofArabidopsis thaliana lack the activity of a basic-amino-acid transporter

Abstract

The concentration dependence of the influx ofl-lysine in excised roots ofArabidopsis thaliana seedlings was analyzed for the wild-type (WT) and two mutants,rlt11 andraec1, which had been selected as resistant to lysine plus threonine, and to S-2-aminoethyl-l-cysteine, respectively. In the WT three components were resolved: (i) a high-affinity, low-capacity component [K _m = 2.2 μM;V _max = 23 nmol·(g FW)⁻¹·h⁻¹]; (ii) a low-affinity, high-capacity component [K _m = 159 μM;V _max = 742 nmol·(g FW)⁻¹·h⁻¹]; (iii) a component which is proportional to the external concentration, with a constant of proportionalityk = 104 nmol·(g FW)⁻¹ h⁻¹];·mM⁻¹. The influx ofl-lysine in the mutants was lower than in the WT, notably in the concentration range 0.1–0.4 mM, where it was only 7% of that in the WT. In both mutants the reduced influx could be fully attributed to the absence of the low-affinity (high-K _m ) component. This component most likely represents the activity of a specific basic-amino-acid transporter, since it was inhibited by several other basic amino acids (arginine, ornithine, hydroxylysine, aminoethylcysteine) but not byl-valine. The high-affinity uptake ofl-lysine may be due to the activity of at least two general amino acid transporters, as it was inhibitable byl-valine, and could be further dissected into two components with a high affinity (K _i = 1–5 μM; and a low affinity (K _i = 0.5–1mM) forl-valine, respectively. Therlt11 andraecl mutant have the same phenotype and the corresponding loci were mapped on chromosome 1, but it is not yet clear whether they are allelic.