Different behaviours of promoters in Mycobacterium tuberculosis H37Rv and H37Ra

Mycobacterium tuberculosis H37Rv and H37Ra are two closely-related bacterial strains in which the former is virulent whereas the latter is not. Although the genetic differences between these strains are known, our understanding of how they control the difference in virulence characteristics is incomplete. In this work, we tested the activities of different mycobacterium gene promoters in the two strains using a gfp reporter gene. The promoter activities were compared between growth in vitro and growth of bacteria residing in U937 cells (a-macrophage-like cell line). The promoters tested included M. tuberculosis isocitrate lyase (icl), alpha crystalline homolog (hspX) and moeZ, and the M. avium macrophage-induced gene (mig) promoter. Two hspX constructs with different lengths of the promoter sequence were tested. All promoters except the shorter hspX construct were active in the H37Ra strain in both liquid culture and in U937 cells. In the H37Rv strain, the shorter hspX and icl constructs were induced in infected U937 cells relative to liquid culture, whereas the mig construct was active in both conditions. In conclusion, the inducible properties of the shorter hspX and the icl promoters evident in H37Rv appeared to be lost in the H37Ra strain. Furthermore, sequence further upstream of the hspX gene appears to modulate the inducible property of the hspX promoter in a strain-dependent manner.