The eight-carbon acid sugar KDO, 3-deoxy-D-manno-2-octulosonic acid, is a primary sugar constituent in various types of cell surface extracellular polysaccharides and liposaccharides of Gram-negative bacteria. In plants, KDO is found only in RG-II. Synthesis of CMP-KDO in plants requires the activities of three enzymes.

1 KdsA, KDO-8-P synthase, catalyzes a condensation of PEP, phosphoenolpyruvate, and phosphorylated monosaccharide, D-arabinose 5-phosphate (A5P) in the presence of metal. Functional genes encoding KDO-8-P synthase activity were isolated from various plant species (498-500) and the encoded proteins share ~50% amino acid sequence iden­tity with the bacterial proteins. In Arabidopsis, two gene isoforms (AtkdsAl, At1g79500; and AtkdsA2, At5g09730) were identified. The encoded isoforms share 93% amino acid sequence identity to each other. Interestingly, AtkdsAl is predominantly expressed in shoots, while AtkdsA2 transcript accumulates to a higher level in roots. The activity of the recombinant plant kdsA toward other phosphorylated-sugars, such as D-erythrose-4- phosphate (E4P) was not tested. Based on bioinformatics, the plant KdsA are predicted to reside in the cytosol.

2 KDO-8-P phosphatase activity removes the phosphate to form KDO. The nature and specificity of this phosphatase is unknown.

3 KdsB, CMP-KDO synthase (CMP-KDOs), catalyzes the transfer of the cytidylyl group (CMP) from CTP to KDO in the production of the unusual nucleotide-sugar, CMP-p-KDO. The resulting activated sugar has a half-life of about 30 minutes in so­lution. The maize gene homolog was functionally identified (501), and the homologous Arabidopsis protein sequence is encoded by At1g53000. In vitro, the recombinant maize CMP-KDO is capable of using both CTP andUTP as nucleotide (Bar-Peled, unpublished). Interestingly, the plant proteins that share 40-50% amino acid sequence identity to the bacterial KdSA proteins have a 50 amino acid N-terminal extension. Bioinformatic anal­ysis suggests that plant CMP-KDO is a Type Ib transmembrane protein with the catalytic domain predicted to face the cytosol. However, the subcellular location of the plant protein is uncertain. Since the function of RGII:2,3KDOT activity was not reported, it remains unclear if CMP or UMP-Kdo are the sugar donors.