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:[1][5]
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
Pyrrolnitrin biosynthesis
Except for prnA, these enzymes are unable to act on D-tryptophan.[1][5]
Neither of the chlorinating enzymes, prnA nor prnC, show homology to known
haloperoxidases nor to one another.[1]
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.[1] 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.