STRUCTURE, SEQUENCE AND EXPRESSION OF THE MOUSE CX43 GENE ENCODING CONNEXIN-43

Gap junctions, membrane channels that mediate the diffusion of ions an d small molecules between cells, are hypothesized to play a role in de velopment and growth regulation. The Cx43 gene (encoding connexin 43) is one member of the gap j unction gene family whose transcripts are e xpressed in a highly regionalized manner during mouse development. We cloned and sequenced Cx43 cDNAs from a 7.5-day mouse embryo cDNA libra ry. These cDNA clones encode the authentic 43-kDa connexin. Analysis o f RNA isolated from different regions of the 7.5-day mouse embryo reve aled that Cx43 transcripts are differentially expressed, with expressi on detected in the embryo proper, but not in the extraembryonic region containing the ectoplacental cone. Using one of the newly isolated mo use Cx43 cDNA probes, we screened a mouse genomic DNA library and clon ed the Cx43 gene. Restriction mapping and sequencing of the cloned gen omic inserts revealed that Cx43 contains two exons and a 10.5-kb intro n located in the 5' untranslated region (5'-UTR). We mapped the Cx43 t ranscription start point (tsp) by RNase protection and primer extensio n analyses and showed that transcripts expressed in the 7.5-day mouse embryo and in adult tissues are initiated from the same tsp. The DNA s equence immediately upstream from the tsp contains a putative AP1-bind ing site and a degenerate TATA consensus sequence. A comparison of mou se, rat, human and bovine Cx43s showed that the 3'-UTR has an unexpect edly high degree of sequence homology. This includes conservation of f our AUUUA motifs, a sequence associated with transcript instability in immediate early genes. However, further analysis of Cx43 expression i n NIH 3T3 and BALB/c 3T3 cells showed that Cx43 expression is not subj ect to serum induction, as has been found for the immediate early gene s.