Abstract
Several prokaryotes have genomes that are densely populated with toxin–antitoxin (TA) loci. For example, the aquatic bloom-forming cyanobacterium Microcystis aeruginosa habours more than 110 TA loci, and the nematode symbiont Photorhabdus luminescens has at least 58 TAs encoded in its genome. Amongst this group of unrelated bacteria lies Mycobacterium tuberculosis whose genome encodes more than 68 TAs. M. tuberculosis causes the devastating human disease tuberculosis (TB) and is, therefore, the subject of intense research. The presence of so many TAs begs the question as to their evolutionary origin, contemporary biological functions and their potential as therapeutic targets in this important human pathogen. This chapter reviews the distribution of TAs in the mycobacterial genus and the experimental results that provide clues to TA function in M. tuberculosis.