Use of the tyrosinase gene from Streptomyces to probe promoter sequences for Escherichia coli

Cloning and sequence analysis of the highly expressed melanin-synthesizing gene operon from Streptomyces castaneoglobisporus

Streptomyces castaneoglobisporus HUT6202 overproduces a diffusible melanin-like pigment. An operon, designated mel, containing a gene that encodes tyrosinase, which is involved in the synthesis of melanin pigment, was cloned from the chromosomal DNA of the microorganism into the high-copy plasmid pAK114 and expressed in S. lividans. The tyrosinase activity of the transformed cells was at approximately a 110-fold higher level than that of the same host carrying the plasmid pIJ702, which has the same replication origin as pAK114 and carries the mel operon from S. antibioticus. The sequence analysis of the S. castaneoglobisporus mel operon revealed that an open-reading frame consisting of 378 base pairs(bp), designated ORF378, was found upstream of the tyrosinase gene (TYRC) consisting of 819 bp. In the present study, we constructed a chimeric mel operon consisting of ORF378 from S. castaneoglobisporus and the tyrosinase gene (TYRA) from S. antibioticus. The chimeric mel operon or the S. antibioticus mel operon, which consists of ORF438 and TYRA, expressed the tyrosinase activity in Escherichia coli intracellularly when located under the control of lacZ promoter, and the tyrosinase activity from the former was at a 30-fold higher level than that from the latter. This suggests that the gene contributing to the high expression of the tyrosinase activity in S. castaneoglobisporus is ORF378, rather than TYRC.

Expression of Streptomyces melC and choA genes by a cloned Streptococcus thermophilus promoter STP2201

Streptococcus thermophilus (ST) chromosomal DNA (chr DNA) fragments having promoter activity were cloned and selected in Escherichia coli using a chloramphenicol acetyltransferase- (cat-) based promoter-probe vector pKK520-3. Insertion of a promoterless streptomycete melanin biosynthesis operon (melC) downstream from the promoters of the library further identified clone STP2201 as a strong promoter in E. coli. Subcloning of a STP2201-melC DNA fragment into the pMEU-series S. thermophilus-E. coli shuttle vectors yielded pEU5xML2201x plasmids that conferred Mel+ phenotype to E. coli. The pEU5aML2201a was further shown to afford a high level of tyrosinase pro-anti-tyrosinase antiserum in S. thermophilus. Substituting melC with a streptomycete cholesterol oxidase gene (choA) in the same orientation yielded pEU5aCH2201a that conferred ChoA activity to an E. coli transformant at a level of (1.06 +/- 0.15) x 10(-7) units mg-1 protein. Introduction of this plasmid into S. thermophilus by electrotransformation yielded ChoA+ transformant that produced the enzyme at about 25% of the level found in E. coli.

Tyrosinase production in recombinant E. coli containing trp promoter and ubiquitin sequence

We have successfully expressed the active tyrosinase of Streptomyces antibioticus in Escherichia coli under the control of the trp promoter by fusing the sequence to the ORF438 gene. Because our attempt to connect the polycistronic gene of ORF438 and tyrosinase directly to the trp promoter of E. coli resulted in the expression of functionally inactive tyrosinase, we decided to fuse the COOH-terminus of ubiquitin sequence to the NH2-terminus of ORF438. Ubiquitin fusion has been shown to augment the yield of cloned gene products in E. coli by increasing the stability or translational efficiency of the fusion proteins. As a result, E. coli transformants harboring a plasmid pTRUBF that contains the ubiquitin-fused ORF438 and the tyrosinase gene produced the strong black pigment of melanin. About 300 units of tyrosinase per liter of batch culture were detected when cultivated in M9 medium containing casamino acids, L-tyrosine, and copper supplements. The black pigment, however, was not seen when grown in LB medium, suggesting that the trp promoter is well regulated. When recombinant E. coli cells grown in LB medium were transferred to a tryptophan-deficient minimal medium with phenol, we observed that phenol was removed from the solution, and the color of the medium turned black. This is due to the fact that the tyrosinase has polyphenol oxidase properties. We expect to use this recombinant E. coli for the waste treatment of phenolic compounds.