Cleavage of beta-carotene as the first step in sexual hormone synthesis in zygomycetes is mediated by a trisporic acid regulated beta-carotene oxygenase.

Carotene cleavage is the necessary initial step in the biosynthesis of trisporic acid, the sexual signal in zygomycete fungi. Two genes encoding putative carotene oxygenases, designated tsp3 and tsp4, were identified in the genome of the zygomycete Rhizopus oryzae. Using heterologous primers, tsp3 was cloned and sequenced also from Blakeslea trispora. tsp3 transcription correlates with sexual development in both species. Northern hybridization of B. trispora mRNA revealed strong induction of tsp3 transcription in mated cultures. A very strong and direct transient induction of transcription by trisporic acid was proven by quantitative real-time PCR analysis. In R. oryzae, transcriptional induction is also inducible by stimulation with trisporoids and depends on the developmental stage of the mycelium. The functionality of the tsp3 gene product as carotene cleavage enzyme was shown as loss of carotene in an Escherichia coli strain transformed to carotene production and tsp3 expression.