TAC from Mycobacterium tuberculosis: a paradigm for stress-responsive toxin-antitoxin systems controlled by SecB-like chaperones

Abstract

Bacterial type II toxin-antitoxins (TAs) are two-component systems that modulate growth in response to specific stress conditions, thus promoting adaptation and persistence. The major human pathogen Mycobacterium tuberculosis potentially encodes 75 TAs and it has been proposed that persistence induced by active toxins might be relevant for its pathogenesis. In this work, we focus on the newly discovered toxin-antitoxin-chaperone (TAC) system of M. tuberculosis, an atypical stress-responsive TA system tightly controlled by a molecular chaperone that shows similarity to the canonical SecB chaperone involved in Sec-dependent protein export in Gram-negative bacteria. We performed a large-scale genome screening to reconstruct the evolutionary history of TAC systems and found that TAC is not restricted to mycobacteria and seems to have disseminated in diverse taxonomic groups by horizontal gene transfer. Our results suggest that TAC chaperones are evolutionary related to the solitary chaperone SecB and have diverged to become specialized toward their cognate antitoxins.

Fig. 1

a To-scale representation of the M. tuberculosis TAC encoding operon. b Distribution of TAC/AC and SecB chaperones across the bacterial taxonomy. Presence of SecB (black, outside circle) or TAC/AC system (gray, inside circle) is depicted as a bar in front of the corresponding leaf in the circular tree of species based on the NCBI taxonomic classification. Leaves are colored according to the taxonomic group. Species discussed in the text are indicated. This representation was generated with iTol

Fig. 2

a MCL analysis of 60 TAC/AC chaperone sequences and 112 solitary SecB chaperone sequences corresponding to the seed sample of the Pfam02256-conserved domain (see Table S1 for abbreviations). The circles (proteins) are colored according to the four communities defined by the program. The links between proteins reflect a BLAST hit with an e-value less than or equal to 10⁻⁵. Abbreviations corresponding to plasmid-borne systems are underlined. b Antitoxin and toxin families found associated with SecB-like chaperones

Fig. 3

The M. smegmatis SecB-like chaperone. a To-scale representation of the genomic island of approximately 15 kb containing the M. smegmatis AC (Msmeg2144-SmegB) genes identified by Alien Hunter. Presence of Che9c phage and transposase genes is indicated. b SmegB replaces SecB in E. coli. Transformants of W3110 ΔsecB containing pSE (vector), pSE-SecB, pSE-Rv1957, or pSE-SmegB were grown to midlog phase, serially diluted and spotted on lysogeny broth (LB)–ampicillin with or without isopropyl β-d-1-thiogalactopyranoside (IPTG), and incubated at the indicated temperature. c SmegB does not replace Rv1957 in TAC control. Midlog phase cultures of W3110 ΔsecB containing the pK6-HigBA1 plasmid and pSE (vector), pSE-Rv1957, or pSE-SmegB were serially diluted and spotted on LB–ampicillin–kanamycin agar plates with or without arabinose and IPTG inducers as indicated. Plates were incubated at 37 °C overnight