Isolation, characterization and developmental expression of the ecdysteroid‐induced E75 gene of the wax moth Galleria mellonella

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Abstract
Using the cDNA for the Drosophila ecdysteroid‐induced member of the steroid‐hormone‐receptor superfamily, E75 A, we isolated a genomic clone from Galleria mellonella that revealed 77% similarity with the region of E75 A cDNA encoding the C‐terminal zinc‐finger motif. A Galleria cDNA clone was isolated that encoded a complete DNA‐binding domain composed of two zinc fingers and designated GmE75 A. Its deduced amino acid sequence showed 100% and 85% identities within the DNA‐binding and ligand‐binding domains of Drosophila E75 A, respectively. The Galleria genomic clone did not encode the N‐terminal zinc finger, but included a sequence similar to the B1 exon, which is unique to the B isoform of E75. Thus, the cDNA and genomic DNA sequences indicated that the Galleria gene E75 encoded at least two isoforms, GmE75 A and GmE75 B, which differed in their N‐termini. Probes specific for GmE75 A and B hybridized to two distinct transcripts of 2.6 kb. Both GmE75 A and B mRNA levels correlated closely with the ecdysteroid titer during development. At the onset of larval/pupal transformation, both transcripts appeared in high amounts within 4 h of the ecdysteroid rise, then declined concurrently with the hormone titer decline. At the time of pupal ecdysis, there was another peak of GmE75 A expression but not GmE75 B expression, coincident with a minor ecdysteroid pulse. In isolated abdomens of final instar larvae, GmE75 A mRNA was induced by 20‐hydroxyecdysone within 20 min of the injection; the mRNA levels were maximal at 1 h and declined by 3 h following the treatment.