This work describes the isolation and characterization of crgA, a Mucor circinelloides gene, which has a dominant-positive effect on light-regulated carotenogenesis. The crgA gene was originally identified in a transformation experiment as a 3'-truncated open reading frame which caused carotenoid overaccumulation in the dark. The complete cloning and sequencing of crgA revealed that its putative product presented several recognizable structural domains: a RING-finger zinc binding domain near the N-terminus, a putative nuclear localization signal, two stretches of acidic amino acids, glutamine-rich regions and a putative isoprenylation motif. The expression of exogenous copies of the complete crgA gene or two different 3'-truncated versions, produced a similar dominant-positive effect on the light-inducible carotenogenesis of M. circinelloides. The presence of these exogenous sequences also caused a missregulation of the endogenous crgA gene, resulting in its overexpression. Collectively, these observations suggest that crgA is involved in the regulation of carotenoid biosynthesis by light.