The Haemophilus influenzae HipBA toxin–antitoxin system adopts an unusual three-component regulatory mechanism

Type II toxin–antitoxin (TA) systems encode two proteins: a toxin that inhibits cell growth and an antitoxin that neutralizes the toxin by direct intermolecular protein–protein interactions. The bacterial HipBA TA system is implicated in persister formation. The Haemophilus influenzae HipBA TA system consists of a HipB antitoxin and a HipA toxin, the latter of which is split into two fragments, and here we investigate this novel three-component regulatory HipBA system. Structural and functional analysis revealed that HipA ^N corresponds to the N-terminal part of HipA from other bacteria and toxic HipA ^C is inactivated by HipA ^N , not HipB. This study will be helpful in understanding the detailed regulatory mechanism of the HipBA ^N+C system, as well as why it is constructed as a three-component system.