Toxin–antitoxin (TA) systems found in diverse bacteria play important roles in their adaptation to changing environments. The toxin of the Fic-1–AntF TA pair from Pseudomonas bijieensis strain 2P24 inhibits bacterial DNA replication by attacking the subunit B of DNA gyrase (GyrB) via AMPylation, while the antitoxin AntF blocks its enzymatic activity by forming a stable protein complex. Although many proteins, including bacterial toxins, have been found to catalyze AMPylation, few enzymes involved in reversing this modification have been described. In this study, we found that the Fic-1–AntF complex functions as a deAMPylase to reverse GyrB modification imposed by Fic-1. Structural and genetic analyses of the Fic-1–AntF complex revealed that Glu28 of AntF is critical for catalysis. Thus, AntF not only functions to inhibit the activity of Fic-1 but also cooperates with the toxin to return the modified substrate to its native form by de-modification. Our results reveal a novel regulatory mechanism for bacterial toxin, which sheds light on the evolution of such enzymes, particularly those of multiple subunits.

Coxiella burnetii, the etiological agent of Q fever, is a significant intracellular bacterial pathogen. C. burnetii is a highly infectious pathogen that primarily targets pulmonary alveolar macrophages during natural infection. It can then disseminate to macrophages in other tissues and organs, leading to chronic infections. C. burnetii is capable of infecting a variety of cultured cells, including primary macrophages, macrophage‐like cells, epithelial cells, and fibroblasts. The virulence of C. burnetii is entirely dependent on the Dot/Icm type IVB secretion system (T4BSS), which delivers effectors into infected cells to modulate cellular pathways for the biogenesis of the Coxiella‐containing vacuole that supports its intracellular replication. A deeper understanding of how C. burnetii exploits host cell processes is essential for developing novel therapeutic strategies to combat infections caused by this important pathogen. This review summarizes the historical milestones and recent advances in our understanding of the structure and function of the C. burnetii Dot/Icm system and its effectors.