A Spontaneous Mutation in tfo X Impairs Bioluminescence in Photobacterium kishitanii ANT ‐2200

Bioluminescence in luminous Vibrionaceae is often regulated by quorum sensing, yet several Photobacterium strains emit light independently of cell density, suggesting alternative regulatory inputs. Here, we characterized DimLum, a spontaneous, stable low‐luminescence mutant of Photobacterium kishitanii ANT‐2200. RNA‐seq showed a pronounced downregulation of the lux–rib operon, while whole‐genome sequencing identified a 20‐nucleotide deletion in tfo X predicted to truncate the C‐terminal region of TfoX. Complementation with the wild‐type tfo X gene restored lux–rib expression and bioluminescence, linking this mutation to the DimLum phenotype. The mutant also displayed a limited transcriptional response, including upregulation of a small set of genes such as a Cro/C1‐type regulator. Together, these results identify TfoX as an important component of the regulatory network supporting bioluminescence in P. kishitanii .