Toxic 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. FitAB binds to its own promoter better than FitA alone. The expected nuclease activity was not observed for the FitAB complex, perhaps because FitA (the antitoxin) prevents metal binding and thus catalysis by FitB. Forms a heterodimer with FitA, 4 FitAB heterodimers form a complex that binds to promoter DNA. The complex is also seen in solution. This protein does not actually contact DNA. Disruption of the fitAB locus 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. Belongs to the PINc/VapC protein family.