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.