The function and metabolic pathway of 3-deoxy-D-manno-octulosonate (KDO) are unclear in plants although it is an essential component in plant cell wall. Here we cloned and characterized a putative Arabidopsis thaliana cytidine monophosphate-KDO synthetase to understand synthetic pathways of KDO. It showed a ubiquitous expression, the activity at an optimal pH of 8.0, and a requirement of Mg2+. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.