DIFFERENCES IN DORSAL AND VENTRAL PIGMENTATION RESULT FROM REGIONAL EXPRESSION OF THE MOUSE AGOUTI GENE

The agouti coat color gene encodes a paracrine signaling molecule that controls the production of yellow and black pigment by melanocytes wi thin hair follicles. Some agouti alleles affect the dorsum and ventrum independently, which has provided the basis for speculation that agou ti gene action in different regions of the body is controlled by disti nct genetic loci that are closely linked. Using a combination of cDNA cloning and RNA expression studies, we find that alternative isoforms of agouti mRNA contain different noncoding first exons located 100 kb apart, whose patterns of expression indicate independent control by re gulatory elements that are either ventral specific or hair cycle speci fic. These results demonstrate that the apparent genetic complexity of the agouti locus is explained by the existence of multiple regulatory elements exerting control over a single coding sequence and provide a conceptual basis for understanding differences in dorsal and ventral hair coloration in many mammalian species. The ventral-specific agouti isoform represents an example of a transcript whose expression is res tricted to ventral skin and provides an approach to investigate the me chanisms by which dorsal-ventral differences in gene expression are es tablished and maintained.