Genomic structure and transcriptional regulation of grass carp calmodulin gene

A fish calmodulin (CaM) gene was characterized for the first time in grass carp. The CaM gene is about 12-Kb in size with identical intron/exon organization as that of mammalian CaM genes. When compared to mammalian counterparts, the 5'-promoter region of grass carp CaM gene contains a TATA box and has a much lower GC content and CpG dinucleotide frequency. Interestingly, the 5'-promoter of carp CaM gene is AT-rich with multiple IRS elements and putative binding sites for Pit-1, Sp1/Sp3 and AP1. Using luciferase reporter assay, a potent silencer region was identified in the distal region of grass carp CaM promoter. Besides, the CaM promoter activity could be upregulated by IGF but suppressed by PACAP, forskolin and over-expression of Sp1 and Sp3. These findings, taken together, indicate that grass carp CaM gene does not exhibit the typical features of housekeeping genes and its expression is under the control of hormone factors, presumably by coupling with the appropriate signaling pathways/transcription factors.