CHARACTERIZATION OF A GENOMIC LOCUS REQUIRED FOR SYNTHESIS OF THE ANTIBIOTIC 2,4-DIACETYLPHLOROGLUCINOL BY THE BIOLOGICAL-CONTROL AGENT PSEUDOMONAS-FLUORESCENS Q2-87

The antibiotic 2,4-diacetylphloroglucinol (Phl) is an important factor in the biological control by fluorescent Pseudomonas spp, of many soi lborne diseases including take-all disease of wheat, A 6.5-kb genomic DNA fragment from Pseudomonas fluorescens Q2-87 conferred production o f Phl and of a red pigment distinct from Phl, but which typically is p resent when Phl is produced, upon all of 13 Phl-nonproducing recipient Pseudomonas strains into which it was introduced, Larger fragments th at included flanking DNA sequences did not transfer this capability, s uggesting that they contain negative regulatory element(s), Analysis o f the 6.5-kb fragment by Tn3HoHo1 mutagenesis further localized the se quences required for Phl production to a segment of approximately 5 kb and revealed the presence of at least two divergently oriented transc riptional 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 p igment, Pleiotropic changes in secondary metabolism or colony morpholo gy were not observed in Pseudomonas strains containing the 6.5-kb frag ment, although some Phl-producing derivatives grew more slowly and gav e rise to smaller colonies than did the wild-type parental strains, Th e size of the genomic region involved in Phl production, and the consi stency and specificity with which these sequences transferred Phl bios ynthetic capability, support the conclusion that the 6.5-kb fragment c ontains the Phl biosynthetic locus.