Four connexin32 (Cx32) cDNA clones isolated from a rat sciatic nerve cDNA library differ in the nucleotide sequence of their 5' untranslated region (UTR) from the corresponding Cx32 cDNA clones previously characterized from liver. The new Cx32 5'UTR sequence detected in the sciatic nerve cDNA clones is identical to one previously found in the 6.5 kb intron of the murine Cx32 gene. Using primer extension and S1 nuclease protection analysis, we determined the transcriptional starting point of this new alternative Cx32 transcript expressed in the sciatic nerve. This starting point is located 444 bp (409 bp) upstream of exon2 in a region previously described as an intron of the Cx32 gene in the rat (and mouse) genome, respectively. The alternative exon1B comprises 99 bp in rat, but 97 bp in the mouse genome, and is spliced to the same exon2 acceptor site also used for splicing of exon1 in liver. Both transcripts are likely to code for the same Cx32 protein whose reading frame is located in exon2. The putative promoter region, upstream of the alternative exon1B, contains a TATAAA motif and has been sequenced and noticed before by Miller et al. (Biosci. Rep. 8, 455-464, (1988)). The alternative exon1B transcript is highly expressed in the sciatic nerve, (i.e. Schwann cells) and very low in liver (i.e. hepatocytes). Its expression is regulated after sciatic nerve injury. The time course of expression was similar to previously established myelin genes and, therefore, we suggest that the expression of the alternative exon1B Cx32 transcript is related to the process of myelination. Very recently, we have characterized another alternative Cx32 exon1A which is transcribed in mouse embryonic stem cells but not in the sciatic nerve (Dahl et al., submitted for publication, 1995). Thus, the murine Cx32 gene is likely to be regulated by three alternative promoters that appear to be activated in a cell type-specific manner.