MOLECULAR-GENETIC CHARACTERIZATION OF 6 RECESSIVE VIABLE ALLELES OF THE MOUSE AGOUTI LOCUS

The agouti locus on mouse chromosome 2 encodes a secreted cysteine-ric h protein of 131 amino acids that acts as a molecular switch to instru ct the melanocyte to make either yellow pigment (phaeomelanin) or blac k pigment (eumelanin). Mutations that up-regulate agouti expression ar e dominant to those causing decreased expression and result in yellow coat color. Other associated effects are obesity, diabetes, and increa sed susceptibility to tumors. To try to define important functional do mains of the agouti protein, we have analyzed the molecular defects pr esent in a series of recessive viable agouti mutations. In total, six alleles (a(mJ) a(u), a(da), a(16H), a(18H), a(e)) were examined at bot h the RNA and DNA level. Two of the alleles, a(16H) and a(e), result f rom mutations in the agouti coding region. Four alleles (a(mJ) a(u), a (18H), and a(da)) appear to represent regulatory mutations that down-r egulate agouti expression. Interestingly, one of these mutations, a(18 H), also appears to cause an immunological defect in the homozygous co ndition. This immunological defect is somewhat analogous to that obser ved in motheaten, (me) mutant mice. Short and long-range restriction e nzyme analyses of homozygous a(18H) DNA are consistent with the hypoth esis that a(18H) results from a paracentric inversion where one end of the inversion maps in the 5' regulatory region of agouti and the othe r end in or near a gene that is required for normal immunological func tion. Cloning the breakpoints of this putative inversion should allow us to identify the gene that confers this interesting immunological di sorder.