Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/1999
Publication Date: N/A
Citation: Interpretive Summary: The biological control agent Pseudomonas fluorescens strain Pf-5, which was originally isolated from a cotton root, produces many antifungal compounds and suppresses plant diseases caused by fungi that live in the soil. One of these compounds is pyoluteorin, which is toxic to the plant pathogenic fungus Pythium ultimum. This manuscript describes the nucleotide sequence of ten genes that encode enzymes for pyoluteorin biosynthesis. The genes are similar to those in other microrganisms that are required for the production of other antibiotics. Based upon their similarities to genes with known function, we ascribed a presumed function to each of the ten described pyoluteorin biosynthesis genes. These functions are consistent with a proposed model for pyoluteorin biosynthesis. We added radioactively-labelled proline to cultures of the bacterium, and demonstrated that proline is a primary precursor of pyoluteorin. We also demonstrated that another gene in the pyoluteorin biosynthesis gene cluster regulates the expression of the biosynthetic genes. This paper is one in a series describing how strain Pf-5 controls plant diseases at the molecular level.
Technical Abstract: Ten genes (plt) required for the biosynthesis of pyoluteorin, an antifungal compound comprised of a bichlorinated pyrrole linked to a resorcinol moiety, were identified within a 24-kb genomic region of Pseudomonas fluorescens Pf-5. Deduced amino acid sequences of eight plt genes were similar to those of genes with known biosynthetic functions, including Type I polyketide synthases (pltB, pltC), an acyl-CoA dehydrogenase (pltE), an acyl-CoA synthetase (pltF), a thioesterase (pltG), and three halogenases (pltA, pltD, and pltM). Insertions of the transposons Tn5 or Tn3-nice or a kanamycin-resistance gene in each of these genes abolished pyoluteorin production by Pf-5. The presumed functions of the eight plt products are consistent with biochemical transformations involved in pyoluteorin biosynthesis from proline and acetate precursors. Isotope labeling studies demonstrated that proline is the primary precursor to the dichloropyrrole moiety of pyoluteorin. The deduced amino acid sequence of the product of another plt gene, pltR, is similar to those of members of the LysR family of transcriptional activators. An operon containing pltR and pltM is transcribed divergently from the pltLABCDEFG gene cluster, and a sequence with the characteristics of a LysR binding site was identified within the 486-bp intergenic region separating pltRM from pltLABCDEFG. Transcription of the pyoluteorin biosynthesis genes pltB, pltE, and pltF, assessed with transcriptional fusions to an ice nucleation reporter gene, was significantly greater in Pf-5 than in a PltR- mutant of Pf-5. Therefore, PltR is proposed to be a transcriptional activator of linked pyoluteorin biosynthesis genes.