Isolation of a functional ecdysteroid receptor homologue from the ixodid tick Amblyomma americanum (L.)

Ecdysteroids are assumed to be the major steroid hormones in arthropods. However, with the exception of insects and crustaceans, very little is known about ecdysteroid action in other arthropods. To determine if ecdysteriods play a functional role in the ixodid tick, Amblyomma americanum (L.), we isolated cDNAs encoding three presumed ecdysteroid receptor isoforms (AamEcRA1, AamEcRA2, and AamEcRA3) that have common DNA and ligand binding domains linked to distinct amino termini. The DNA and ligand binding domains share an average of 86 and 64% identity, respectively with DNA and ligand binding domains from insect EcR proteins. The amino termini are highly divergent and the AamEcRs lack the 'F' domain found in the insect EcRs. Analysis of AamEcR cDNAs show that processing of the AamEcR gene is complex, producing multiple transcripts with unique 5' and 3' termini as well as splicing variants with incomplete open reading frames. AamEcR mRNA profiles in whole animals and isolated tissues are consistent with complex regulation of AamEcR expression. We also examined the ability of AamEcRA1, when paired with an AamRXR, to activate transcription of an ecdysone response element containing reporter, and demonstrate that the AamEcR gene encodes a functional ecdysteroid receptor.