| Description | The seven genes that form the core of this apparently laterally transferred cassette include four genes homologous to the NeuABCD genes producing acetyl-modified CMP-neuraminic (sialic) acids. This nine-carbon sugar is formed from the condensation of UMP-GlcNAc and phosphoenolpyruvate. In addition to these genes are an epimerase/dehydratase gene distantly similar to 4,6-dehydratases and a aminotransferase distantly similar to perosamine synthetase. It is reasonable to propose that these two genes act on UDP-GlcNAc or a similar sugar to remove the 6-hydroxyl group and replace the 4-hydroxyl group with an amino group, possibly changing the chirality of one or both of those centers in the process. The purpose of the final additional gene in the cassette, a nucleotidyltransferase is unclear, although the production of a bis-nucleotidylsugar, or alternatively, two different nucleotidylsugars are possibilities. In many organisms additional genes are observed intercalated with these seven, suggesting additional modifications to the product such as methyl- and formyl-transferases, redox enzymes, etc. |
