Genomic organization of DHR38 gene in Drosophila: presence of Alu-like repeat in a translated exon and expression during embryonic development.

The recombinant lambda clone 4-2 containing the genomic region of the Drosophila hormone receptor 38 (DHR 38) gene, homologous to mammalian neuronal growth factor I-B (NGFI-B), was isolated by radioactive labelled oligonucleotide hybridization. The nucleotide sequence of the genomic clone revealed three exons that encode the functional domains of the protein. The N-terminal exon1 had no homology at the amino acid level with NGFI-B, the mammalian homologue. A glutamine-rich region, probably involved in transcriptional activation, was observed at the C-terminal part of this exon. A similar motif is also present upstream in another reading frame of the same strand. Both motifs are preceded by a repetitive non-anucleotide sequence containing an AluI site, resembling a duplicated human Alu-sequence. A monomeric version of this sequence, coding similarly for an oligoglutamine peptide, occurs at a surprisingly high frequency in other regulatory genes in Drosophila. In contrast to mammalian Alu sequences, this sequence is found almost exclusively in the coding regions of Drosophila genes, but not in the non-coding parts of the genes. The DNA-binding domain with two zinc-fingers, and at least part of the ligand-binding peptide, is coded by the largest middle exon2 in DHR38 and exhibits up to 100% homology in short peptide motifs to its mammalian counterpart, where these domains are split into exons 3, 4, 5, and 6. However, the length, information content, stop codon, and splice site are conserved in the last exons in both fly and rat. In situ hybridization to 0-24 h wholemount embryos showed strong expression of DHR38 in neurogenic regions and in the intestinal tract during embryogenesis, suggesting a spatial and temporal control of transcription, partially analogous to the central nervous system-specific expression of NGFI-B in mammals.