Genomic structure and developmental expression of the chicken nonocarboxylate transporter MCT3 gene

MCT3 is a monocarboxylate transporter that is specifically expressed on the basolateral membrane of retinal pigment epithelial cells (RPE). In these studies the temporal expression of MCT3 during ocular development was examined using Northern blot analysis. A 2.2 kb transcript (MCT3b) was detected in RPE by embryonic day 7 (E7) and was present throughout embryonic development. A 2.45 kb transcript (MCT3a) was expressed at low levels before E11 but its expression increased between E11 and E17. Using 5'-RACE (rapid amplification of cDNA ends) it has demonstrated that MCT3a and MCT3b mRNA had distinct 5'-untranslated sequences but shared the same translation start site. To determine the exon-intron structure and to understand the elements that control the tissue specific and developmental expression of MCT3, the MCT3 gene was cloned and sequenced from a chicken genomic library. The MCT3 gene is distributed over 8 kb of DNA and is composed of 6 exons. Coding sequences for MCT3 are found on exon 2 through exon 5. Comparison of the 5'-RACE sequence with the genomic sequence reveals that the two 5'-untranslated regions of the mRNAs are encoded by distinct exons, 1a and 1b, which are alternatively spliced to exon 2. These data suggest that two forms of MCT3 mRNAs could be generated by two distinct promoters that may be regulated in response to changes in the metabolic activity of the retina during development.