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Mutations that enhance the response to double-stranded RNA (dsRNA) have revealed components of the RNA interference (RNAi) pathway or related small RNA pathways. To explore these small RNA pathways, we screened for Caenorhabditis elegans mutants displaying an enhanced response to exogenous dsRNAs. Here we describe the isolation of mutations in two adjacent, divergently transcribed open reading frames (eri-6 and eri-7) that fail to complement. eri-6 and eri-7 produce separate pre-messenger RNAs (pre-mRNAs) that are trans-spliced to form a functional mRNA, eri-6/7. Trans-splicing of eri-6/7 is mediated by a direct repeat that flanks the eri-6 gene. Adenosine to inosine editing within untranslated regions of eri-6 and eri-7 pre-mRNAs reveals a double-stranded pre-mRNA intermediate, forming in the nucleus before splicing occurs. The ERI-6/7 protein is a superfamily I helicase that both negatively regulates the exogenous RNAi pathway and functions in an endogenous RNAi pathway. A screen in C. elegans roundworms for mutations that enhance the response to exogenous double-stranded RNA (dsRNA) has resulted in a surprising discovery: the assembly of an RNA interference gene by trans-splicing of its two component parts via a dsRNA intermediate. The product of the resulting intermediate is the helicase, ERI-6/7, which functions as a negative regulator of exogenous and endogenous RNAi. This is one of very few examples of trans-splicing in metazoans and perhaps significantly, the first instance of an RNAi factor so regulated.

In contrast with the higher order of plants (Spermophyta), essential oils are rare in all the lower orders (Thallophyta, Bryophyta and Pteridophyta)1. There are only four records among the Pteridophyta, and in none of these is there any definite evidence for the presence of terpenes or their derivatives.