Aconitic acid, an unsaturated tricarboxylic acid, is used in the chemical industry as raw materials for organic synthesis, especially as a specific substrate for a flavoring agent. trans-Aconitic acid (tAA) is a trans-isomer of cis-aconitic acid and detected in some plants and bacteria. However, biosynthetic route and metabolism of tAA in relation to assimilation have been unknown. Aconitate isomerase (AI; EC 53.3.7) catalyzes the reversible isomerization between cisaconitic acid and tAA. Pseudomonas sp. WU-0701 was isolated as a bacterium assimilating tAA as sole carbon source, and characterization and gene identification of AI were already reported. Here, we describe that Pseudomonas sp. WU-0701 exhibited growth in each synthetic medium containing glucose, citric acid, isocitric acid, or tAA as sole carbon source. AI was intracellularly detected all the time during the cultivation of the strain WU-0701 cells, irrespective of the carbon sources; AI activity was detected even in the glucose-grown cells. Through the subcellular fractionation experiments, AI was detected in the periplasmic fraction. This is the first report indicating that a bacterium belonging to the genus Pseudomonas is constitutive for the AI production. (C) 2020, The Society for Biotechnology, Japan. All rights reserved.