Genomic structure and promoter analysis of the mouse EphA8 receptor tyrosine kinase gene

The gene encoding the mouse EphA8 receptor tyrosine kinase has been isolated from a mouse genomic library, and its complete genomic structure has been determined. This gene spans approximately 28 kb and consists of 17 exons. This gene structure is similar to the structure of the chick EphB2 (Cek5) gene, except for one intron present between the first two exons encoding the EphA8 kinase domain. This difference may reflect an evolutionary divergence of the catalytic domain between EphA and EphB subgroup receptors. The site for transcription initiation has been mapped to the 19th nucleotide upstream from the translation start codon ATG. A feature of this gene is an unmethylated CpG island spanning exon 1 and the flanking sequence. The putative promoter of the EphA8 gene lacks a TATA box and contains multiple copies of the sequence GGGCGG, the core sequence of the putative Sp1-binding site. The 3.5-kb upstream genomic region containing part of the first exon showed strong promoter activity in NG108-15 neuroblastoma cells but much less in 293T cells, suggesting that this fragment is sufficient for neural cell-directed promoter activity. By deleting the genomic region containing the five GC boxes, it was shown that the minimal promoter region is primarily comprised of five copies of the Sp1-binding site located upstream from the transcription initiation site. Finally, in situ RNA hybridization studies revealed a very specific pattern of EphA8 gene expression restricted to the rostral region of midbrain tectum during embryonic development. Isolation of a functional promoter for the EphA8 gene is a first step in understanding how expression of this gene is controlled at the molecular level.