The level of a transcript required for production of a Streptomyces coelicolor antibiotic is conditionally dependent on a tRNA gene.

In Streptomyces coelicolor A3(2), bldA mutants are conditionally defective in aerial mycelium formation and fail to synthesize all four antibiotics produced by bldA+ strains. Previous studies showed that bldA specifies the tRNA for the rarely used leucine codon UUA. Here we describe experiments examining the abundance in a bldA mutant of a transcript involved in antibiotic production. With use of a bacteriophage-based integrative vector, a promotorless xylE reporter gene was inserted into a previously undescribed gene for an early step in biosynthesis of the red antibiotic undecylprodigiosin, located in the red gene cluster. With this transcriptional fusion present at unit copy number in the chromosome, xylE expression in a bldA+ strain was maximal late in growth in a liquid production medium and was virtually absent in a bldA mutant. On plates of a different medium, the bldA mutant was able to produce undecylprodigiosin and to express the red::xylE fusion, but both abilities were repressed by increasing the concentration of phosphate in the medium. These experiments showed that the undecylprodigiosin deficiency of bldA mutants cannot be accounted for by the presence of TTA codons in the red structural genes, but rather that bldA influences red gene mRNA abundance. In low-phosphate conditions, an alternative regulatory pathway can lead to red gene expression.