A Spontaneous Mutation in tfoX 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 tfoX predicted to truncate the C-terminal region of TfoX. Complementation with the wild-type tfoX 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.