Pyrrolnitrin (PRN) is a naturally occurring

phenylpyrrole Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, in ...

fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...

. ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...

'' and ''

Burkholderia ''Burkholderia'' is a genus of Pseudomonadota whose pathogenic members include the ''Burkholderia cepacia'' complex, which attacks humans and plants; ''Burkholderia mallei'', responsible for glanders, a disease that occurs mostly in horses and r ...

'' species produce pyrrolnitrin from

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromat ...

as secondary metabolite. It is believed that the antifungal properties come from inhibition of electron transport system. The synthetic fungicides fenpiclonil and

fludioxonil Fludioxonil is a synthetic phenylpyrrole chemical introduced by Ciba-Geigy (now Syngenta) in 1993 for use as a non-systemic fungicide. It is a structural analog of the natural fungicide pyrrolnitrin. It is used for the treatment of crops, particu ...

are chemically related to pyrrolnitrin.

Biosynthesis

In ''Pseudomonas fluorescens'', biosynthesis of pyrrolnitrin requires four

genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

, named ''prnABCD,'' arranged into a single

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

''.'' 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), in a process requiring

NAD NAD or Nad may refer to: Geography * Nad, County Cork, a village in Ireland * North American Datum, a series of geographic coordinate systems * North Atlantic Drift, an Atlantic Ocean current * Hobli, a subdivision of a taluka in southern Ind ...

* prnB – ring rearrangement and

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

of 7-chloro-L-tryptophan to form monodechloroaminopyrrolnitrin (MAD) * prnC – chlorination of monodechloroaminopyrrolnitrin to form aminopyrrolnitrin (APRN), in a process also requiring

* prnD – oxidation of the

amino group In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

of APRN to a nitro group thus completing the biosynthesis 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 The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and archaea and a ''N''-formylmethionine (fMet) in bacteria, mitochondria and plastids. ...

, further lowering the amount of prnB expressed and thus lowering the amount of present APP.

References

{{reflist Antifungals Pyrroles Chlorobenzene derivatives Nitrobenzene derivatives Halogen-containing alkaloids