The two mammalian a2,8-polysialyltransferases(polyST's), ST8Sia II (STX) and ST8Sia IV (PST), catalyzesynthesis of the a2-8-linked polysialic acid (polySia) glycanson neural cell adhesion molecules (NCAMs). The objective ofthis study was to clone the coding sequence of the pigletST8Sia II and determine the mRNA expression levels ofST8Sia II, ST8Sia IV, NCAM and neuropilin-2 (NRP-2), alsoa carrier protein of polySia, during postnatal development.The amino acid sequence deduced from the coding sequenceof ST8Sia II was compared with seven other mammalian species.Piglet ST8Sia II was highly conserved and shared 67.8%sequence identity with ST8Sia IV. Genes coding for ST8Sia IIand IV were differentially expressed and distinctly different inneural and non-neural tissues at postnatal days 3 and 38.Unexpectedly, the cellular levels of mRNA coding forST8Sia II and IV showed no correlation with the posttranslationallevel of polySia glycans in different tissues. In contrast,mRNA abundance coding for NCAM and neuropilin-2 correlatedwith expression of ST8Sia II and IV. These findingsshow that the cellular abundance of ST8Sia II and IV in postnatalpiglets is regulated at the level of translation/posttranslation, and not at the level of transcription, a findingthat has not been previously reported. These studies furtherhighlight differences in the molecular mechanisms controllingpolysialylation in adult rodents and neonatal piglets.