A core-promoter region functions bi-directionally for human opioid-receptor-like gene ORL1 and its 5′-adjacent gene GAIP

We report the genomic structure and functional activities of the promoter regions of the human opioid-receptor-like gene ORL1 and its 5′-adjacent gene GAIP (G alpha interacting protein). The transcription and alternative splicing of human ORL1 are controlled by two alternate promoters, located approximately 10 kb apart. The two promoter regions lack a TATA-box and are GC rich. Promoter 1A initiates, from a single transcription start point (TSP), two transcripts: one consisting of exons 1A, 1B, 2, etc., the other without exon 1B. A potential ATG codon upstream of the initiation codon of ORL1 starts a new open-reading frame encoding a theoretical polypeptide of 205 amino acid residues. The promoter 1B transcribes, from multiple TSPs, only one mRNA starting with exon 1B. Two different repeat sequence polymorphisms are found in the ORL1 promoter regions. Luciferase reporter gene assays with promoter regions and a series of deletion mutants have mapped the core-promoter 1A and 1B within two short fragments. DNA sequencing and a database search reveal that the human GAIP gene is located upstream of ORL1 and is oriented in the opposite direction. The transcription and alternative splicing of GAIP are also under the control of alternate promoters. The first exons of ORL1 and GAIP are separated by only 83 bp. This 83 bp fragment, together with short surrounding sequences from both first exons, functions bi-directionally as a core-promoter for both genes. The transcription and alternative splicing of human ORL1 and GAIP are cell-type specific. While GAIP is expressed in both NT2 precursor cells and differentiated NT2 neuronal cells, ORL1 is only expressed in differentiated NT2 neurons. Since ORL1 is a G protein-coupled receptor and GAIP interacts with G protein α subunits, their physical linkage in the genome and co-operative transcriptional regulation may play a significant role in ORL1 receptor signal transduction.