Involved in biosynthesis of legionaminic acid (5,7-diamino-3,5,7,9-tetradeoxy-D-glycero-D-galacto-non-2-ulosonic acid)(Leg), a sialic acid-like derivative that is incorporated into virulence-associated cell surface glycoconjugates such as lipopolysaccharide (LPS) which could be a key determinant in the ability of L.pneumophila to inhibit the fusion of phagosomes with lysosomes. LPS contains a majority alpha2,4-linked homomer of legionaminic acid. Catalyzes the conversion of N,N'-diacetyllegionaminic acid (Leg5Ac7Ac) and CTP into CMP-N,N'-diacetyllegionaminic acid (CMP-Leg5Ac7Ac). CTP + N,N-diacetyllegionaminate = CMP-N,N-diacetyllegionaminate + diphosphate Belongs to the CMP-NeuNAc synthase family.