TSPO safeguards porphyrin–iron balance under anoxic conditions in Bacillus cereus

The physiological role of the bacterial translocator protein (TSPO) under anoxic conditions remains poorly defined. Here, we show that deletion of tspO in Bacillus cereus growing under fermentative anoxic conditions leads to increased porphyrin accumulation, redox imbalance, and impaired growth. Cellular proteome analyses reveal extensive remodeling of central metabolism, including increased abundance of proteins in the siroheme branch of tetrapyrrole biosynthesis, increased abundance of nitrate assimilation and respiratory nitrate reduction components, and broad alterations in iron, metal ion, and phosphate homeostasis. The ΔtspO mutant also displays a characteristic nitrosative stress signature, with sustained induction of NO-detoxifying systems. These combined defects result in marked changes in the exoproteome. Taken together, our findings support a model in which B. cereus TSPO links porphyrin metabolism to intracellular iron utilization, thereby supporting heme biosynthesis and redox balance under anoxic conditions.IMPORTANCEUnderstanding how bacteria maintain metabolic and redox balance under anoxic conditions is essential for deciphering how they adapt to fluctuating environments. Here, we identify the translocator protein Bacillus cereus TSPO (BcTSPO) as a key coordinator of porphyrin and iron metabolism in Bacillus cereus, contributing to the proper functioning of the heme biosynthetic pathway and limiting the accumulation of toxic intermediates. Loss of BcTSPO triggers redox and nitrosative stress, ion dysregulation, and broad metabolic instability, highlighting its central role in maintaining cellular homeostasis during oxygen deprivation. These findings extend the physiological scope of bacterial TSPO beyond oxygen-dependent contexts and support the idea that its ancestral function lies in coordinating tetrapyrrole metabolism with intracellular iron use to sustain cellular resilience across diverse environmental conditions.