The initial step in the conversion of the isoprenoid intermediate farnesyl diphosphate to the sesquiterpenoid phytoalexin capsidiol in elicitor-treated tobacco tissues is catalyzed by an inducible sesquiterpene cyclase [5-epiaristolochene synthase (EAS)]. Two independent cDNA clones (cEAS1 and cEAS2) encoding EAS were isolated from an elicitor-induced tobacco cDNA library by differential hybridization and subsequently were characterized by hybrid selection-in vitro translation. Insertion of cEAS1, a partial cDNA clone encoding 175 C-terminal amino acids, into an Escherchia coli expression vector resulted in accumulation of a fusion protein immunodetectable with EAS-specific polyclonal antibodies. The cDNA clones were used to isolate two full-length EAS genes that mapped 5 kilobases (kb) apart on one 15-kb genomic clone. The nucleotide sequences of the structural gene components were identical from 388 base pairs (bp) upstream of the transcription initiation site to 40 bp downstream of the translation termination codon, suggesting a relatively recent duplication event. The genes consist of 1479-bp open reading frames, each containing five introns and specifying 56,828-Da proteins. The N-terminal amino acid sequence deduced from the genomic clones was identical to the first 16 amino acids of the EAS protein identifuible by Edman degradation. RNA blot hybridization with cEAS1 demonstrated a mRNA induction time course consistent with the induction of the EAS mRNA translational activity with maximum levels 4-6 h after elicitation. EAS mRNA was not detected in control cells. DNA blot-hybridization analysis of genomic DNA revealed a copy number of '12-15 for EAS-like genes in the tetraploid tobacco genome. The conservation of a putative allelic prenyl diphosphate binding motif is also discussed.Sesquiterpenoids are a structurally diverse class of isoprenoids found in plants, some fungi, and bacteria, but not in vertebrates, which have important implications in plantplant (1), plant-insect (2, 3), and plant-pathogen (4, 5) interactions. Despite an extensive appreciation for the structure-function relationships of plant sesquiterpenoids, the sesquiterpenoid biosynthetic pathway and its regulation are not well understood (4, 6). Although many enzymes of general isoprenoid biosynthesis have been measured in plants, few enzymes suspected of being rate-limiting have been identified (4, 6). Moreover, little is known about the mechanisms regulating the activity and absolute levels of plant isoprenoid biosynthetic enzymes with the exception of a diterpene cyclase (7). This is in contrast to the extensive molecular analysis of regulatory mechanisms controlling isoprenoid metabolism in mammalian systems (8).Tobacco cell suspension cultures respond to treatment with elicitors such as fungal cell wall hydrolysates or cellulase by the de novo synthesis and secretion of antibiotic sesquiterpenoids, primarily the phytoalexin capsidiol (9, 10). Central to capsidiol biosynthesis is the induction of a sesquiterpene cyclase [5-epi...