Genomic Organization of Connexins
The general genomic structure of connexin genes is rather simple. The 5′-untranslated region (5′-UTR) of connexin mRNA is part of exon 1 that is separated from exon 2 by a long intron. Exon 2 harbours the entire open reading frame and the subsequent 3′-untranslated region (3′-UTR). However, with the accumulation of new experimental data, it became evident that the genomic organization of several connexin genes refutes this simplicity. The identification of different splice isoforms of several connexin genes demonstrated that exon 1 harbouring the 5′-UTR could be spliced in an alternate manner possibly due to alternate promoter usage. However, it should be emphasized that these transcript isoforms vary only in their 5′-untranslated region, leaving the coding region unaltered. Moreover, there are well-known exceptions to the above-proposed genomic structure. For example, Cx36, Cx39, and Cx57 coding regions are interrupted by another intron. The variation in the genomic organization of various connexin genes can be ascertained by the following paragraphs that describe the structure of several individual connexin genes.