Highly fluorescent GFP_m²⁺-based genome integration-proficient promoter probe vector to study Mycobacterium tuberculosis promoters in infected macrophages

Abstract

Study of activity of cloned promoters in slow-growing Mycobacterium tuberculosis during long-term growth conditions in vitro or inside macrophages, requires a genome-integration proficient promoter probe vector, which can be stably maintained even without antibiotics, carrying a substrate-independent, easily scorable and highly sensitive reporter gene. In order to meet this requirement, we constructed pAKMN2, which contains mycobacterial codon-optimized GFP_m²⁺ gene, coding for GFP_m²⁺ of highest fluorescence reported till date, mycobacteriophage L5 attP-int sequence for genome integration and a multiple cloning site. pAKMN2 showed stable integration and expression of GFP_m²⁺from M. tuberculosis and M. smegmatis genome. Expression of GFP_m²⁺ driven by the cloned minimal promoters of M. tuberculosis cell division gene, ftsZ (MtftsZ), could be detected in the M. tuberculosis/pAKMN2-promoter integrants, growing at exponential phase in defined medium in vitro and inside macrophages. Stable expression from genome-integrated format even without antibiotic, and high sensitivity of detection by flow cytometry and fluorescence imaging, in spite of single copy integration, make pAKMN2 useful for the study of cloned promoters of any mycobacterial species under long-term in vitro growth or stress conditions, or inside macrophages.