tailieunhanh - Báo cáo khoa học: Purification and characterization of glutamate N-acetyltransferase involved in citrulline accumulation in wild watermelon

Citrulline is an efficient hydroxyl radical scavenger that can accumulate at concentrations of up to 30 mmin the leaves of wild watermelon during drought in the presence of strong light; however, the mechanism of this accumulation remains unclear. In this study, we characterized wild water-melon glutamateN-acetyltransferase (CLGAT) that catalyses the trans-acetylation reaction between acetylornithine and glutamate to form acetylglutamate and ornithine, thereby functioning in the first and fifth steps in citrulline biosynthesis | ềFEBS Journal Purification and characterization of glutamate -acetyltransferase involved in citrulline accumulation in wild watermelon Kentaro Takahara Kinya Akashi and Akiho Yokota Graduate Schoolof BiologicalSciences Nara Institute of Science and Technology Japan Keywords citrulline drought strong-light stress glutamate N-acetyltransferase thermostability wild watermelon Correspondence A. Yokota Nara Institute of Science and Technology Graduate Schoolof Biological Sciences 8916-5 Takayama Ikoma Nara 630-0101 Japan Fax 81 743 72 5569 Tel 81 743 72 5560 E-mail yokota@ Received 12 July 2005 revised 18 August 2005 accepted 23 August 2005 doi Citrulline is an efficient hydroxyl radical scavenger that can accumulate at concentrations of up to 30 mM in the leaves of wild watermelon during drought in the presence of strong light however the mechanism of this accumulation remains unclear. In this study we characterized wild watermelon glutamate N-acetyltransferase CLGAT that catalyses the transacetylation reaction between acetylornithine and glutamate to form acetylglutamate and ornithine thereby functioning in the first and fifth steps in citrulline biosynthesis. CLGAT enzyme purified 7000-fold from leaves was composed of two subunits with different N-terminal amino acid sequences. Analysis of the corresponding cDNA revealed that these two subunits have molecular masses of and kDa and are derived from a single precursor polypeptide suggesting that the CLGAT precursor is cleaved autocatalytically at the conserved ATML motif as in other glutamate N-acetyltransferases of microorganisms. A green fluorescence protein assay revealed that the first 26-amino acid sequence at the N-terminus of the precursor functions as a chloroplast transit peptide. The CLGAT exhibited thermostability up to 70 C suggesting an increase in enzyme activity under high leaf temperature conditions during drought strong-light stresses. Moreover .