Three prokaryotic-derived probes to identify and study the temporal expression of oligo-or poly(sialic acid) also recognized rat brain membranes, further substantiating the presence of poly(sialic acid) in rat brain. This conclusion was confirmed by using a mutant of E. coli K1 that was defective in the synthesis of poly(sialic acid) and could only transfer sialic acid to exogenous acceptors of oligo-or poly(sialic acid). Sialyl polymer synthesis was restored in the mutant when brain membranes were added as exogenous acceptor.Sialic acid occurs primarily as the terminal, nonreducing sugar on N-asparaginyl-linked glycoproteins often attached to galactosyl residues of bi-, tri-, or tetraantennary sugar chains. Rarely, however, sialic acid exists internally to form polysialosyl chains of various lengths. A well-characterized poly(sialic acid) is a capsular polysaccharide, the K1 anti-