Pyrrolnitrin

Pyrrolnitrin (PRN ) is a phenylpyrrole chemical used an antifungal antibiotic. Pseudomonas pyrrocinia and other Pseudomonas species produce pyrrolnitrin from tryptophan as secondary metabolite. It is believed that the antifungal properties come from inhibition of electron transport system.

The synthetic fungicides fenpiclonil and fludioxonil are chemically related to pyrrolnitrin.

Biosynthesis
In Pseudomonas fluorescens, biosynthesis of pyrrolnitrin requires four genes, named prnABCD, arranged into a single operon. The products of these genes are similar in size and catalyze four subsequent reactions:


 * prnA – chlorination of L -tryptophan to 7-chloro- L -tryptophan (7-CLT), requiring NAD for its activity
 * prnB – ring rearrangement and decarboxylation of 7-chloro- L -tryptophan to form monodechloroaminopyrrolnitrin (MAD)
 * prnC – chlorination of monodechloroaminopyrrolnitrin to form aminopyrrolnitrin (APRN), requiring NAD for its activity
 * prnD – oxidation of amino group to form nitro group of pyrrolnitrin



Except for prnA, these enzymes are unable to act on D -tryptophan.

Neither of the chlorinating enzymes, prnA nor prnC, show homology to known haloperoxidases nor to one another.

An alternative pathway was also suggested, where L -tryptophan is first turned into aminophenylpyrrole (APP) and then by subsequent steps to aminopyrrolnitrin and pyrrolnitrin. While these steps have not been described in more detail, prnB is able to produce APP, presumably from tryptophan as starting material. APP seems to be an unwanted side product. The gene coding for prnB also starts with the unusual GTG start codon, further lowering the amount of prnB expressed and thus lowering the amount of present APP.