Urocanase is inactivated in intact cells of Pseudomonas putida and photoactivated by brief exposure of the cells to the UV radiation in sunlight. The dark reversion (inactivation) in vitro is explained by the formation of a sulfite-NAD adduct. Our objective was to investigate the dark reversion in vivo. Various compounds were added to P. putida cells, and the reversion was measured, after sonication, by comparison of the activity before and after UV irradiation. Sulfite, cysteine sulfinate, and hypotaurine enhanced the reversion of urocanase in resting cells. The reversion was time and concentration dependent. Sulfite modified the purified enzyme, but cysteine sulfinate and hypotaurine could not, indicating that those two substances had to be metabolized to support the reversion. Both of those compounds yielded sulfite when they were incubated with cells. Transaminases form sulfite from cysteine sulfinate. P. putida extract contained a transaminase whose activity involved as alpha-keto acid and either cysteine sulfinate or hypotaurine for (i) production of sulfite, (ii) disappearance of substrates, (iii) formation of corresponding amino acids, and (iv) urocanase reversion. Porcine crystalline transaminase caused reversion of highly purified P. putida urocanase with cysteine sulfinate and alpha-ketoglutarate. We conclude that in P. putida cysteine sulfinate or hypotaurine is catabolized in vivo by a transaminase reaction to sulfite, which modifies urocanase to a form that can be photoactivated. We suggest that this photoregulatory process is natural because it occurs in cells with the aid of sunlight and cellular metabolism.