The positive activator of cephamycin C and clavulanic acid production in Streptomyces clavuligerus is mistranslated in a bldA mutant

In Streptomyces coelicolor bldA encodes the principal leucyl tRNA for translation of UUA codons and controls pigmented antibiotic production by the presence of TTA codons in the genes encoding the pathway-specific activators of actinorhodin and undecylprodigiosin biosynthesis. In Streptomyces clavuligerus the gene encoding the pathway-specific activator of both cephamycin C and clavulanic acid production, ccaR, also contains a TTA codon and was expected to exhibit bldA-dependence. A cloned S. clavuligerus DNA fragment containing a sequence showing 91% identity to the S. coelicolor bldA-encoded tRNA was able to restore antibiotic production and sporulation to bldA mutants of S. coelicolor and the closely related Streptomyces lividans. A null mutation of the bldA gene in S. clavuligerus resulted in the expected sporulation defective phenotype, but unexpectedly had no effect on antibiotic production. Transcript analysis showed no difference in the levels of ccaR transcripts in the wild-type and bldA mutant strains, ruling out any effect of elevated levels of the ccaR mRNA. Furthermore, when compared to the wildtype strain, the bldA mutant showed no differences in the levels of CcaR, suggesting that the single TTA codon in ccaR is mistranslated efficiently. The role of codon context in bldA dependence is discussed.