TTA codons in some genes prevent their expression in a class of developmental, antibiotic-negative, Streptomyces mutants.

Abstract

In Streptomyces coelicolor A3(2) and the related species Streptomyces lividans 66, aerial mycelium formation and antibiotic production are blocked by mutations in bidA, which specifies a tRNAL'-like gene product which would recognize the UUA codon. Here we show that phenotypic expression of three disparate genes (carB, lacZ, and ampC) containing TTA codons depends strongly on bWd. Site-directed mutagenesis of carB, changing its two TTA codons to CTC (leucine) codons, resulted in bld4-independent expression; hence the bi4 product is the principal tRNA for the UUA codon. Two other genes (hyg and aad) containing TTA codons show a medium-dependent reduction in phenotypic expression (hygromycin resistance and spectinomycin resistance, respectively) in bM4 mutants. For hyg, evidence is presented that the UUA codon is probably being translated by a tRNA with an imperfectly matched anticodon, giving very low levels of gene product but relatively high resistance to hygromycin. It is proposed that TTA codons may be generally absent from genes expressed during vegetative growth and from the structural genes for differentiation and antibiotic production but present in some regulatory and resistance genes associated with the latter processes. The codon may therefore play a role in developmental regulation.

Isolation and characterization of a strong promoter element from the Streptomyces ghanaensis phage I19 using the gentamicin resistance gene (aacC1) of Tn1696 as reporter

“…Inspection of the nucleotide sequence of aacC1 (Wohlleben et al, 1989) reveals that 41 % of codons possess adenine or thymine in the third position, compared to a mean of only 9 YO for 64 analysed Streptomyces genes (Bibb et al, 1984;Wright 8c Bibb, 1992). Furthermore, TTA codons are located at positions two and nine of the aacCl coding region, and there is evidence to suggest that the translation of these rare codons may be limiting in vegetatively growing cultures (Leskiw et al, 1991a, b). In S. coelicolor A3(2) and in S. griseus, the efficient translation of UUA codons requires the leucyl-tRNA encoded by 6ldA (Piret & Chater, 1985;McCue et al, 1992).…”

A relA/spoT Homologous Gene from Streptomyces coelicolor A3(2) Controls Antibiotic Biosynthetic Genes

“…The bld (bldA-D and bldF-G) (21,22) genes have been described as being required for both antibiotic production and aerial mycelium formation. The bldA gene encodes a leucyltRNA (23) that recognizes the UUA codon (extremely rare in Streptomyces mRNA because of the high G ϩ C content of their DNA), and the suggestion has been made that this gene might constitute a translational regulatory mechanism controlling sporulation genes and some antibiotic pathways (9,24).…”

The variant tuf3 gene of Streptomyces coelicolor A3(2) encodes a real elongation factor Tu, as shown in a novel Streptomyces in vitro translation system

“…Investigation of the specific residues participating in formation of an EF-Tu/aa-tRNA complex [4] emphasizes again the unique character of EF-Tu3, which contains substitutions at conserved positions in the binding sites for the aminoacylated 3 H end and the T-stem of aa-tRNA. In this context it is interesting to note that Streptomyces species contain a special tRNA Leu for translation of the very rare codon UUA, which is encoded by the bldA gene and is pleiotropically involved in morphological differentiation and antibiotic biosynthesis [33,34]. The unique structural feature of this leucyl-tRNA is the lack of a GG sequence in the d-loop [35], that is generally conserved in all tRNAs involved in cytoplasmic protein biosynthesis.…”