Characterization of a Genomic Locus Required for Synthesis of the Antibiotic 2,4-diacetylphloroglucinol by the Biological Control Agent Pseudomonas fluorescens Q2-87. Gita M. Bangera. Department of Microbiology, Washington State University, Pullman 99164 U.S.A. Linda S.Thomashow(1,2). (1)Department of Microbiology, Washington State University, Pullman 99164 U.S.A. and (2) United States Department of Agriculture, Agricultural Research Service, Root Disease and Biological Control Research Unit, Pullman, WA 99164-6430 U.S.A. MPMI 9:083-090. Accepted 2 November 1995, This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1996.

The antibiotic 2,4-diacelylphlorogIucinol (Phi) is an important factor in the biological control by fluorescent Pseudomonas spp. of many soilborne diseases including take-all disease of wheat. A 6.5-kb genomic DNA fragment from Pseudomonas fluorescens Q2-87 conferred production of Phl and of a red pigment distinct from Phl, but which typically is present when Phl is produced, upon all of 13 Phl-nonproducing recipient Pseudomonas strains into which it was introduced. Larger fragments that included flanking DNA sequences did not transfer this capability, suggesting that they contain negative regulatory element(s). Analysis of the 6.5-kb fragment by Tn3HoHol mutagenesis further localized the sequences required for Phl production to a segment of approximately 5 kb and revealed the presence of at least two divergently oriented transcriptional units. Insertions within the smaller unit or within about 3 kb of the 5' end of the larger unit caused loss of production of both Phl and the red pigment. Other insertions within the distal 1.5 kb of the larger transcriptional unit abolished production of only the red pigment. Pleiotropic changes in secondary metabolism or colony morphology were not observed in Pseudomonas strains containing the 6.5-kb fragment, although some Phi-producing derivatives grew more slowly and gave rise to smaller colonies than did the wild-type parental strains. The size of the genomic region involved in Phi production, and the consistency and specificity with which these sequences transferred Phl biosynthetic capability, support the conclusion that the 6.5-kb fragment contains the Phl biosynthetic locus.