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

Benzoic Acid-Inducible Gene Expression in Mycobacteria

Conditional expression is a powerful tool to investigate the role of bacterial genes. Here, we adapt the Pseudomonas putida-derived positively regulated XylS/Pm expression system to control inducible gene expression in Mycobacterium smegmatis and Mycobacterium tuberculosis, the causative agent of human tuberculosis. By making simple changes to a Gram-negative broad-host-range XylS/Pm-regulated gene expression vector, we prove that it is possible to adapt this well-studied expression system to non-Gram-negative species. With the benzoic acid-derived inducer m-toluate, we achieve a robust, time- and dose-dependent reversible induction of Pm-mediated expression in mycobacteria, with low background expression levels. XylS/Pm is thus an important addition to existing mycobacterial expression tools, especially when low basal expression is of particular importance.

Controlling gene expression in mycobacteria

The genus mycobacterium contains some of the most important human pathogens, including Mycobacterium tuberculosis, which causes tuberculosis in approximately 8 million people annually; Mycobacterium leprae, the etiologic agent of leprosy, which affects millions of people in Asia, Africa and Latin America; and Mycobacterium bovis, which causes tuberculosis in animals and people. Genetic tools for mycobacteria have been developed during the last 15 years and have helped to improve our understanding of the biology and pathogenesis of mycobacteria. However, genetic switches have only recently been developed that allow control of mycobacterial gene expression. Such systems have been used to facilitate protein overexpression in mycobacteria and to analyze gene function, in particular functions of essential genes. Some of the recently developed systems will allow controlling gene expression during animal infections and may therefore become invaluable tools for drug target validation. This article will review the features of available mycobacterial genetic switches and discuss their applications.

Differential green fluorescent protein expression from mycobacterial promoter constructs in Escherichia coli and Mycobacterium marinum

The Mycobacterium marinum G13 promoter is a sigma 70-like promoter that is more active by green fluorescent protein (gfp) differential fluorescence induction (DFI) assays when M. marium resides in an intracellular compartment as compared with growth in media. In assays using DFI, we found that the mycobacterial G13 promoter was also more active in a background of lower nutrient availability during logarithmic growth. This promoter, contained in an insert cloned upstream of a gfp reporter gene, is also active in Escherichia coli. When gfp expression assays of different plasmid constructs were performed in parallel with E. coli and M. marinum, expression in E. coli was maintained after deletion of both upstream and/or downstream regions proximal to the core promoter sequence. In M. marinum, however, although upstream deletions had no appreciable effect on gfp expression, promoter constructs with deleted downstream regions expressed 20- to 40-fold less gfp over all growth phases. The high-level expression of gfp was restored, however, in a clone containing approximately 100 bp downstream of the transcriptional start point. We have therefore utilized this gfp reporter assay of promoter activity to distinguish possible differences in requirements for gfp expression between different genera that utilize sigma 70-like promoter elements. We found that high levels of expression of gfp from the G13 promoter in M. marinum require downstream regions not necessary for gfp expression in E. coli.

Actinomycetes as host cells for production of recombinant proteins

Actinomycetes (Actinobacteria) are highly attractive as cell factories or bioreactors for applications in industrial, agricultural, environmental, and pharmaceutical fields. Genome sequencing of several species of actinomycetes has paved the way for biochemical and structural analysis of important proteins and the production of such proteins as recombinants on a commercial scale. In this regard, there is a need for improved expression vectors that will be applicable to actinomycetes. Recent advancements in gene expression systems, knowledge regarding the intracellular environment, and identification and characterization of plasmids has made it possible to develop practicable recombinant expression systems in actinomycetes as described in this review.

Tetracycline-inducible gene regulation in mycobacteria

A system for the tetracycline-inducible regulation of gene expression in mycobacteria has been developed. We have sub-cloned the tetRO region from the Corynebacterium glutamicum TetZ locus into a mycobacterial shuttle plasmid, making expression of genes cloned downstream of tetRO responsive to tetracycline. Using the luxAB-encoded luciferase from Vibrio harveyi as a reporter (pMind-Lx), we observed a 40-fold increase in light output from Mycobacterium smegmatis cultures 2 h after adding 20 ng ml⁻¹ of tetracycline. Similarly, exposure to the drug resulted in up to 20-fold increase in relative light units from M.bovis BCG carrying the reporter construct, and a 10-fold increase for M.tuberculosis. Tetracycline induction was demonstrated in log and stationary phase cultures. To evaluate whether this system is amenable to use in vivo, J774 macrophages were infected with M.bovis BCG[pMind-Lx], treated with amikacin to kill extracellular bacteria, and then incubated with tetracycline. A 10-fold increase in light output was measured after 24 h, indicating that intracellular bacteria are accessible and responsive to exogenously added tetracycline. To test the use of the tetracycline-inducible system for conditional gene silencing, mycobacteria were transformed with a pMind construct with tetRO driving expression of antisense RNA for the ftsZ gene. Bacterial cells containing the antisense construct formed filaments after 24 h exposure to tetracycline. These results demonstrate the potential of this tetracycline-regulated system for the manipulation of mycobacterial gene expression inside and outside cells.

Controlling gene expression in mycobacteria with anhydrotetracycline and Tet repressor

Gene expression systems that allow the regulation of bacterial genes during an infection are valuable molecular tools but are lacking for mycobacterial pathogens. We report the development of mycobacterial gene regulation systems that allow controlling gene expression in fast and slow-growing mycobacteria, including Mycobacterium tuberculosis, using anhydrotetracycline (ATc) as inducer. The systems are based on the Escherichia coli Tn10-derived tet regulatory system and consist of a strong tet operator (tetO)-containing mycobacterial promoter, expression cassettes for the repressor TetR and the chemical inducer ATc. These systems allow gene regulation over two orders of magnitude in Mycobacterium smegmatis and M.tuberculosis. TetR-controlled gene expression was inducer concentration-dependent and maximal with ATc concentrations at least 10- and 20-fold below the minimal inhibitory concentration for M.smegmatis and M.tuberculosis, respectively. Using the essential mycobacterial gene ftsZ, we showed that these expression systems can be used to construct conditional knockouts and to analyze the function of essential mycobacterial genes. Finally, we demonstrated that these systems allow gene regulation in M.tuberculosis within the macrophage phagosome.

Expression systems for use in actinomycetes and related organisms

There have been significant advances in genetic and molecular approaches to understanding the physiology of organisms belonging to the genera Mycobacterium, Corynebacterium, Nocardia and Streptomyces. This review discusses recent advances in heterologous protein expression in members of the actinomycete group, including codon usage, post-translational modification and inducible gene expression.