Sia also sporadically occur in procaryotic microorganisms. While some of those have a biosynthetic
machinery of their own, others acquire Sia from the
environment (Vimr and Lichtensteiger, 2002). Since all
enzymes involved in these processes are similar to those
in animal cells, both in their properties and gene
structures, it appears possible that these have been
acquired from host cells by natural gene transfer
(Schauer and Vliegenthart, 1982).