Isolation and characterization of the human homologue of rig and its pseudogenes: the functional gene has features characteristic of housekeeping genes.

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Abstract
Rig (rat insulinoma gene) was first isolated from a cDNA library of rat insulinomas and has been found to be activated in various human tumors such as insulinomas, esophageal cancers, and colon cancers. Here we isolated the human homologue of rig from a genomic DNA library constructed from a human esophageal carcinoma and determined its complete nucleotide sequence. The gene is composed of about 3000 nucleotides and divided into four exons separated by three introns: exon 3 encodes the nuclear location signal and the DNA-binding domain of the RIG protein. The transcription initiation site was located at -46 base pairs upstream from the first ATG codon. The 5'-flanking region of the gene has no apparent TATA-box or CAAT-box sequence. However, two GC boxes are found at -189 and -30 base pairs upstream from the transcription initiation site and five GC boxes are also found in introns 1 and 2. The gene is bounded in the 5' region by CpG islands, regions of DNA with a high GC content and a high frequency of CpG dinucleotides relative to the bulk genome. Furthermore, the human genome contains at least six copies of RIG pseudogenes, and four of them have the characteristics of processed pseudogenes. From these results together with the finding that RIG is expressed in a wide variety of tissues and cells, we speculate that RIG belongs to the class of "housekeeping" genes, whose products are necessary for the growth of all cell types.