Inducibility of the Streptomyces traRts107-Ptra expression cassette in Mycobacterium smegmatis

An inducible Streptomyces expression cassette utilising the Ptra promoter and a temperature-sensitive allele of the TraR repressor from S. nigrifaciens plasmid pSN22 was tested in Mycobacterium smegmatis. Using reporter assays and Northern blot analysis, a marked increase of Ptra-directed transcription was observed upon a temperature shift from 28 to 37 degrees C. These results show that the S. nigrifaciens promoter-repressor cassette is functional in M. smegmatis. However, comparison of the level of induced Ptra-directed transcription with the level of transcription directed by the strong mycobacterial promoter Phsp60 indicated that the relative strength of the Ptra promoter was low. Considering the severe limitation of inducible expression systems for mycobacteria, this Streptomyces cassette might be a useful starting point for the development of a compact and fully portable inducible mycobacterial expression cassette.