Abstract
Citrulline is a compatible solute in wild watermelon, a drought-tolerant plant native to the Kalahari Desert, Africa. Citrulline is an intermediary amino acid in the arginine biosynthetic pathway, and has a potent activity for scavenging hydroxyl radicals. In wild watermelon, accumulation of citrulline in the leaves is triggered by the onset of drought stress in the presence of high light. Previous results showed that the activity of N-acetylglutamate kinase (AGK), a key enzyme in the citrulline biosynthetic pathway, was significantly up-regulated in the leaves under the stress. In this study, a cDNA for this enzyme, designated CLAGK, and a cDNA for the putative regulator of AGK, CLPII, were isolated from wild watermelon. Deduced amino acid sequences of both proteins had putative chloroplast transit peptides at their N-terminus, and showed 67-79% deduced amino acid sequence similarity with those of corresponding genes reported from Arabidopsis. Moreover, western blot analysis showed that abundance of CLPII was markedly up-regulated in the leaves under drought. We are currently performing kinetic analysis of CLAGK and CLPII using their recombinant proteins.
