CLONING AND BACTERIAL EXPRESSION OF SESQUITERPENE CYCLASE, A KEY BRANCH POINT ENZYME FOR THE SYNTHESIS OF SESQUITERPENOID PHYTOALEXIN CAPSIDIOL IN UV-CHALLENGED LEAVES OF CAPSICUM-ANNUUM

Sesquiterpene cyclase, a branch point enzyme in the general isoprenoid pathway for the synthesis of phytoalexin capsidiol, was induced in de tached leaves of Capsicum annuum (pepper) by UV treatment. The inducib ility of cyclase enzyme activities paralleled the absolute amount of c yclase protein(s) of pepper immunodetected by monoclonal antibodies ra ised against tobacco sesquiterpene cyclase. A cDNA library was constru cted with poly(A)(+) RNA isolated from 24 h UV-challenged leaves of pe pper. A cDNA clone for sesquiterpene cyclase in pepper was isolated by using a tobacco 5-epi aristolochene synthase gene as a heterologous p robe. The predicted protein encoded by this cDNA was comprised of 559 amino acids and had a relative molecular mass of 65,095, The primary s tructural information from the cDNA clone revealed that it shared 77%, 72% and 49% identity with 5-epi aristolochene, vetispiradiene, and ca dinene synthase, respectively, The enzymatic product catalyzed by the cDNA clone in bacteria was identified as 5-epi aristolochene, as judge d by argentation TLC, RNA blot hybridization demonstrated the inductio n of an mRNA consistent with the induction of cyclase enzyme activity in UV-treated pepper.