Identification of two distinct promoters in the chicken caldesmon gene

Caldesmon (CaD) is a suitable molecular marker for phenotypic modulation of smooth muscle cells. Chicken CaD gene is composed of 17 exons with a whole length of 100-150 kilobases (kb). Exons 1a-1, 1a-2, and 1a-3 encode the 5'-terminal sequence specific to mRNA for gizzard type CaD, and exon 1b encodes the sequence specific to brain type CaD mRNA. Here, we have characterized the 5'-upstream regions of chicken CaD gene. Primer extension analysis revealed that the transcriptional starting sites of gizzard and brain CaD mRNAs were 218 and 279 nucleotides upstream from each translational initiation codon, respectively. We have identified two distinct promoters (gizzard type and brain type promoters) in the CaD gene by bacterial chloramphenicol acetyltransferase (CAT) assay using chick embryo fibroblasts (CEFs). The 5'-upstream region of exon 1a-1 showed remarkable promoter activity, but the activity of the 5'-upstream region of exon 1b was low (10% of the former). These results indicate that CaD subtypes are generated by differential RNA transcriptions.