Antitoxin component of a type II toxin-antitoxin (TA) system. Plays a role in the speed with which bacteria traverse human epithelial cells; disruption of the locus increases the speed of trafficking about 2-4-fold. Binds to its own promoter, binding affinity of the FitAB complex is 20-30-fold higher than FitA alone. No nuclease activity was observed for the FitAB complex, perhaps because FitA (the antitoxin) prevents metal binding and thus catalysis by FitB. Homodimer in the absence of FitB; forms a heterodimer with FitB; 4 FitAB heterodimers form a complex that binds to fitAB promoter DNA. The complex is also seen in solution. Disruption of the fitAB operon leads to faster transepithelial cell trafficking of the bacterium; mutants adhere to and invade cells normally. Mutants grow normally in liquid culture but much faster within human cell lines A431 and T84; these latter 2 phenotypes were observed using MS11A bacteria with a disrupted fitAB locus.