Except for prnA, these enzymes are unable to act on D-tryptophan.
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.
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Kirner, Sabine; Hammer, Philip E.; Hill, D. Steven; Altmann, Annett; Fischer, Ilona; Weislo, Laura J.; Lanahan, Mike; van Pée, Karl-Heinz; Ligon, James M. (April 1998). "Functions Encoded by Pyrrolnitrin Biosynthetic Genes from Pseudomonas fluorescens". Journal of Bacteriology. 180 (7): 1939–1943. doi:10.1128/JB.180.7.1939-1943.1998. ISSN 0021-9193. PMC 107110. PMID 9537395. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC107110
De Laurentis, Walter; Khim, Leang; Anderson, J. L. Ross; Adam, Ariane; Phillips, Robert S.; Chapman, Stephen K.; van Pee, Karl-Heinz; Naismith, James H. (2007-10-01). "The Second Enzyme in Pyrrolnitrin Biosynthetic Pathway Is Related to the Heme-Dependent Dioxygenase Superfamily". Biochemistry. 46 (43): 12393–12404. doi:10.1021/bi7012189. ISSN 0006-2960. PMC 3326534. PMID 17924666. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3326534
Kirner, Sabine; Hammer, Philip E.; Hill, D. Steven; Altmann, Annett; Fischer, Ilona; Weislo, Laura J.; Lanahan, Mike; van Pée, Karl-Heinz; Ligon, James M. (April 1998). "Functions Encoded by Pyrrolnitrin Biosynthetic Genes from Pseudomonas fluorescens". Journal of Bacteriology. 180 (7): 1939–1943. doi:10.1128/JB.180.7.1939-1943.1998. ISSN 0021-9193. PMC 107110. PMID 9537395. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC107110
De Laurentis, Walter; Khim, Leang; Anderson, J. L. Ross; Adam, Ariane; Phillips, Robert S.; Chapman, Stephen K.; van Pee, Karl-Heinz; Naismith, James H. (2007-10-01). "The Second Enzyme in Pyrrolnitrin Biosynthetic Pathway Is Related to the Heme-Dependent Dioxygenase Superfamily". Biochemistry. 46 (43): 12393–12404. doi:10.1021/bi7012189. ISSN 0006-2960. PMC 3326534. PMID 17924666. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3326534
Kirner, Sabine; Hammer, Philip E.; Hill, D. Steven; Altmann, Annett; Fischer, Ilona; Weislo, Laura J.; Lanahan, Mike; van Pée, Karl-Heinz; Ligon, James M. (April 1998). "Functions Encoded by Pyrrolnitrin Biosynthetic Genes from Pseudomonas fluorescens". Journal of Bacteriology. 180 (7): 1939–1943. doi:10.1128/JB.180.7.1939-1943.1998. ISSN 0021-9193. PMC 107110. PMID 9537395. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC107110
Kirner, Sabine; Hammer, Philip E.; Hill, D. Steven; Altmann, Annett; Fischer, Ilona; Weislo, Laura J.; Lanahan, Mike; van Pée, Karl-Heinz; Ligon, James M. (April 1998). "Functions Encoded by Pyrrolnitrin Biosynthetic Genes from Pseudomonas fluorescens". Journal of Bacteriology. 180 (7): 1939–1943. doi:10.1128/JB.180.7.1939-1943.1998. ISSN 0021-9193. PMC 107110. PMID 9537395. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC107110