Figure 1.

MazF toxins can specifically target mRNA, rRNA, or tRNA. All mazEF operons encode the intracellular toxin MazF downstream of the gene for the antitoxin MazE. In the absence of stress, the MazE antitoxin neutralizes the toxicity of MazF by forming a stable complex with the toxin. The MazEF complex and to a lesser extent, the MazE dimer, autoregulate the operon. Thus, the amount of free, active MazF toxin is dynamic and dependent on the concentration of the unstable MazE antitoxin that is susceptible to the action of cellular proteases. Many MazF family members appear to regulate growth by cleaving single-stranded mRNA at unique and specific 3-, 5- or 7-base sequences. However, E. coli MazF also cleaves 16S rRNA in the 30S ribosomal subunit. M. tuberculosis toxins MazF-mt6 and MazF-mt3 also cleave 23S rRNA in the 50S subunit or both 16S and 23S rRNA, respectively. By contrast, M. tuberculosis MazF-mt9 cleaves tRNA.