Genomic organization and the 5' upstream sequences associated with the specific spatio-temporal expression of HrEpiC, an epidermis-specific gene of the ascidian Halocynthia roretzi.

An epidermis-specific gene HrEpiC of the ascidian Halocynthia roretzi is activated in all presumptive blastomeres by the 64-cell stage. To explore the molecular mechanisms underlying the regulation of the timing of activation of HrEpiC, we studied the genomic organization and the 5' upstream sequences of HrEpiC associated with specific spatio-temporal expression of the gene. The restriction site mapping and sequencing of genomic clones showed that the H. roretzi genome contained two copies of HrEpiC gene, HrEpiC1 and HrEpiC2, aligned tandemly in about 8 kb of the genome. Analysis of various deletion constructs with the 5' flanking sequences of HrEpiC1 revealed that 103 bp of the 5' flanking region was sufficient for the minimal epidermis-specific expression of HrEpiC1 and that the region between -281 bp and -198 bp of the 5' flanking region was associated with the amplification of the minimal expression of the reporter gene in the epidermis. This module between -281 bp and -198 bp was also shown to be associated with the timing of the activation of HrEpiC1 by the 64-cell stage. We discussed how the spatio-temporal expression pattern of HrEpiC1 is regulated by the two modules.