Catalyzes the production of the sugar nucleotide CMP-3-deoxy-D-manno-octulosonate (CMP-KDO) (PubMed:10829033). CTP is the preferred nucleotide donor, and it can partially be replaced with UTP but not with ATP (PubMed:10829033). Activates KDO during the biosynthesis of rhamnogalacturonan II (RG-II), a structurally complex pectic polysaccharide of the primary cell wall (By similarity). RG-II is essential for the cell wall integrity of rapidly growing tissues and pollen tube growth and elongation (By similarity). 3-deoxy-alpha-D-manno-oct-2-ulosonate + CTP = CMP-3-deoxy-beta-D-manno-octulosonate + diphosphate Optimum pH is 9.5. Nucleotide-sugar biosynthesis; CMP-3-deoxy-D-manno-octulosonate biosynthesis; CMP-3-deoxy-D-manno-octulosonate from 3-deoxy-D-manno-octulosonate and CTP: step 1/1. Ubiquitous. Rhamnogalacturonan II (RG-II) RG-II is a structurally complex pectic polysaccharide present in the primary cell wall. RG-II consists of a linear 1,4-linked a-Dgalacturonic acid backbone with four distinct side chains, two of which contain apiosyl residues. Boric acid forms a diester with two apiosyl residues from separate RG-II molecules, thereby covalently cross-linking two RG-II molecules as a dimer. Belongs to the KdsB family.