Transcriptome-wide identification, characterization and phylogenomic analysis of cytochrome P450s from Nothapodytes nimmoniana reveal candidate genes involved in camptothecin biosynthetic pathway

The plant Nothapodytes nimmoniana is a source of important anticancer compound camptothecin (CPT) which is being used in the treatment of colorectal, lung and ovarian cancers. Camptothecin is biosynthesized by the combination of seco-iridoid and indole pathway in plants. Majority of the biosynthetic steps and associated genes still remains mysterious. Few reactions in the seco-iridoid pathway are catalysed by cytochrome P450 enzymes. Hence, identifying transcriptionally active cytochrome P450 genes becomes essential in the elucidation of CPT biosynthetic pathway. Here, we report the identification of 94 cytochrome P450s from the assembled transcriptomic data of leaf and root of N. nimmoniana. The identified cytochrome P450s were full length and possessing all four conserved characteristic signature motifs of cytochrome P450 genes. Phylogenetic analysis of protein sequences revealed evolution, diversification and further categorized them into A type (52.12%) and non-A type (47.87%) cytochrome P450s. These 94 sequences represent 38 families and 63 subfamilies of cytochrome P450s. We also compared transcriptional activity of identified cytochrome P450s with the expression of their homologs in CPT producing plant Ophiorrhiza pumila. Based on expression profile and validation using quantitative PCR, we propose NnCYP81CB1 and NnCYP89R1 as the candidate cytochrome P450 genes involved in camptothecin biosynthesis in N. nimmoniana.