Noncognate Mycobacterium tuberculosis Toxin-Antitoxins Can Physically and Functionally Interact

Abstract

The Mycobacterium tuberculosis genome harbors a striking number (>40) of toxin-antitoxin systems. Among them are at least seven MazF orthologs, designated MazF-mt1 through MazF-mt7, four of which have been demonstrated to function as mRNA interferases that selectively target mRNA for cleavage at distinct consensus sequences. As is characteristic of all toxin-antitoxin systems, each of the mazF-mt toxin genes is organized in an operon downstream of putative antitoxin genes. However, only one of the seven putative upstream antitoxins (designated MazE-mt1 through MazE-mt7) has significant sequence similarity to Escherichia coli MazE, the cognate antitoxin for E. coli MazF. Interestingly, the M. tuberculosis genome contains two independent operons encoding E. coli MazE orthologs, but they are not paired with mazF-mt-like genes. Instead, the genes encoding these two MazE orthologs are each paired with proteins containing a PIN domain, indicating that they may be members of the very large VapBC toxin-antitoxin family. We tested a spectrum of pair-wise combinations of cognate and noncognate Mtb toxin-antitoxins using in vivo toxicity and rescue experiments along with in vitro interaction experiments. Surprisingly, we uncovered several examples of noncognate toxin-antitoxin association, even among different families (e.g. MazF toxins and VapB antitoxins). These results challenge the "one toxin for one antitoxin" dogma and suggest that M. tuberculosis may enlist a sophisticated toxin-antitoxin network to alter its physiology in response to environmental cues.

MazF ribonucleases promote Mycobacterium tuberculosis drug tolerance and virulence in guinea pigs

“…We did not observe any cross-talk between these non-cognate MazEF protein pairs, in contrast to previous reports (Fig. 1f) 43 . RT-PCR analysis demonstrated the differential induction of the mazEF TA systems, and under most of the conditions tested, the levels of antitoxins transcripts were lower than the levels of their cognate toxin transcripts (Fig.…”

“…2). In contrast to previous reports 43 , we did not observe any interaction between non-cognate MazF toxins and MazE antitoxins (Fig. 1f).…”

Regulatory crosstalk between type I and type II toxin-antitoxin systems in the human pathogen Enterococcus faecalis

“…12,45,55,56 Recent studies have demonstrated that TA-II systems can affect positively or negatively the expression of other TA-II modules. 39,57,58 However in these studies, only changes in steady-state RNA levels have been shown, and RNA synthesis (transcription) and RNA stability effects cannot be distinguished. In our experiments, direct RNA stability measurements rule out stability effects of MazE and MazEF on RatA, since the RatA half-life is independent of MazE and MazEF levels.…”

“…A few reports show that TA-II modules coordinate their expression by acting on their respective RNA levels or by direct interactions between non-cognate toxins and antitoxins. 39,58 Modeling such interactions suggests that interplays between TAs offer better chances to create heterogeneous populations. 59 Thus, it is tempting to speculate that functional and regulatory crosstalks between TAs, associated to genomic plasticity may provide the E. faecalis species with capacities to conquer habitats and adapt life styles.…”

Latent Tuberculosis Infection: Myths, Models, and Molecular Mechanisms

“…There are 8 major families of TA modules: CcdBA, HigBA, HipBA, MazEF, ParDE, RelBE, VapBC, and Doc/PhD (291). A recent study revealed that 88 putative TA system candidates are present in M. tuberculosis, considerably more than in any other intracellular pathogen, potentially allowing bacilli to respond and adapt to the various stresses encountered during infection and rendering the bacilli refractory to antibiotics until conditions for growth are more favorable (292).…”