Methanococcus jannaschii generates L-proline by cyclization of L-ornithine.

Cell extracts of Methanococcus jannaschii have been shown to readily convert L-ornithine to L-proline. This cyclization reaction proceeds with the loss of only the C-2 nitrogen, as has been documented for ornithine cyclodeaminase (EC 4.3.1.12). Since no gene homologous to that coding for ornithine cyclodeaminase is present in the genome of M. jannaschii, these results indicate that proline biosynthesis in M. jannaschii is accomplished by a previously unrecognized enzyme.