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A number of essential genes have been identified in mycobacteria, but methods to study these genes have not been developed, leaving us unable to determine the function or biology of the genes. We investigated the use of a tetracycline-inducible expression system in Mycobacterium tuberculosis and Mycobacterium smegmatis. Using a reporter gene which encodes an unstable variant of GFP, we showed that tetracycline-inducible expression occurred in M. smegmatis and that expression levels were titratable to some extent by varying the concentration of tetracycline. The removal of tetracycline led to cessation of GFP expression, and we showed that this was a controllable on/off switch for fluorescence upon addition and removal of the antibiotic inducer. The system also functioned in M. tuberculosis, giving inducible expression of the reporter gene. We used homologous recombination to construct a strain of M. tuberculosis that expressed the only copy of the tryptophan biosynthetic enzyme, TrpD, from the tetracycline-inducible promoter. This strain was conditionally auxotrophic, showing auxotrophy only in the absence of tetracycline, confirming that trpD was tightly controlled by the foreign promoter. This is the first demonstration of the use of an inducible promoter to generate a conditional auxotroph of M. tuberculosis. The ability to tightly regulate genes now gives us the possibility to define the functions of essential genes by switching them off under defined conditions and paves the way for in vivo studies.

The acetamidase of Mycobacterium smegmatis is an inducible enzyme which enables the organism to utilise several amides as sole carbon sources. The acetamidase structural gene (amiE) is located downstream of four other genes, of which three form a probable operon with amiE; the fourth (amiC) is divergently transcribed. We constructed deletion mutants in two of these genes in order to determine their role in acetamidase expression. Both AmiC and AmiD were shown to be positive regulators of acetamidase expression required for induction. Combinations of regulatory gene deletions were made which revealed that AmiC interacts with the previously characterised negative regulator AmiA, whereas AmiD does not.

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