Identification of the RLBP1 gene promoter

Cellular retinaldehyde-binding protein (CRALBP), transcribed from the RLBP1 gene, is a 36-kDa water-soluble protein with 316 amino acids found in the retinal pigment epithelium (RPE) and in retinal Müller cells. It is thought to play a critical role in the visual cycle by functioning as an acceptor of 11-cis-retinol from the isomerohydrolase reaction. The goal here was to evaluate the functional promoter of this gene.

5' RACE analysis, promoter-reporter assays, and semiquantitative PCR with exon-specific primers were performed using human-derived RPE cells (ARPE-19 and D407) in culture to evaluate the 5' sequence flanking the RLBP1 gene. In addition, the murine, bovine, and porcine RLBP1 genes were evaluated in silico to identify likely proximal promoter/exon 1 sequences similar to the human gene.

5' RACE analysis revealed the presence of a previously undescribed exon in the RLBP1 gene. This was confirmed by analysis of the GenBank Human EST database, which revealed the presence of 18 sequences matching exon 1. Exon-specific PCR revealed that most CRALBP transcripts expressed in ARPE-19 cells contain both exon 1 and the final exon, suggesting that the primary promoter of CRALBP exists 5' of the newly identified exon 1. Highly homologous sequences in the murine, bovine, and porcine genes were also identified. Finally, promoter-reporter constructs revealed a minimal sequence necessary for promoter function and indicated significantly greater promoter activity compared with previously described RLBP1 promoters.

The findings presented here suggest that CRALBP transcripts in RPE cells contain a noncoding exon in addition to a newly described promoter and, by definition, an additional intron. This finding sets the stage for a mechanistic understanding of the high degree of cell type-specific expression of RLBP1.