Abstract
Drought/strong-light stresses cause an accumulation of citrulline, an efficient hydroxyl radical scavenger, in wild watermelon leaves to 30 mM. To understand the mechanism for its accumulation, we characterized glutamate N-acetyltransferase (GAT) involved in the simultaneous reactions of the first and fifth steps in the citrulline biosynthesis. The 7000-fold-purified GAT enzyme from the watermelon leaves was composed of two different polypeptides. Analysis of the corresponding cDNA cloned revealed that the two polypeptides were derived from the single precursor polypeptide, suggesting that the GAT precursor is autocatalytically cleaved like other GATs from microorganisms. The green fluorescence protein assay revealed that the first 26 amino acid sequence at the N-terminus of the precursor functions as the chloroplast transit peptide. The GAT showed thermostability up to 70οC and was not inhibited by citrulline and arginine at physiologically-relevant high concentrations unlike Arabidopsis GAT. We discuss contribution of GAT to citrulline accumulation during drought/strong-light stresses.
