Involvement of the 5’-UTR of afsS in the activation of actinorhodin production in Streptomyces lividans

afsS was originally found as a gene that conferred the production of actinorhodin on Streptomyces lividans, when it was carried on a high copy number plasmid. In Streptomyces coelicolor A3(2), transcription of afsS is activated by AfsR, which is a pleiotropic regulator for secondary metabolism in this species. afsS encodes a 63-amino-acid protein containing three repeats of the sequence Thr-X₂-Asp-Asn-His-Met-Pro-X₂-Pro-Ala (X: non-conserved amino acids). Although the Asp and His residues in the repeated sequences in AfsS have been shown to be important for the enhancement of actinorhodin production in S. coelicolor A3(2), how AfsS stimulates the transcription of actII-ORF4 (which encodes the pathway-specific transcriptional activator for actinorhodin production) and how overexpression of afsS awakes “sleeping” actinorhodin production in S. lividans have been unknown. Here, I report an unexpected function of the 5’ untranslated region (5’-UTR) of afsS in the activation of actinorhodin production in S. lividans. The afsS mRNA has a 5’-UTR (144 nucleotides), which could form a rigid hairpin structure with a loop of seven nucleotides (UACGGGG). Only this 5’-UTR was enough to confer the actinorhodin production on S. lividans, when it was overexpressed. Furthermore, replacement of any one of the 3’ five nucleotides (CGGGG) of the loop sequence abolished the function of 5’-UTR, while the nucleotide sequence of the stem region was not important as far as it formed a similar stem structure. Together with other results, the functions of 5’-UTR and coding sequence of afsS in awaking “sleeping” actinorhodin production in S. lividans are discussed.