THE EMBRYONIC LETHALITY OF HOMOZYGOUS LETHAL YELLOW MICE (A(Y) A(Y)) IS ASSOCIATED WITH THE DISRUPTION OF A NOVEL RNA-BINDING PROTEIN/

Lethal yellow (A(y)) is a mutation at the mouse agouti (a) locus that is associated with an all-yellow coat color, obesity, diabetes, tumors in heterozygotes, and preimplantation embryonic lethality in homozygo tes. Previously, we cloned and characterized the wild-type agouti gene and demonstrated that it expresses a 0.8-kb mRNA in neonatal skin. In contrast, A(y) expresses a 1.1-kb transcript that is ectopically over expressed in all tissues examined. The A(y) mRNA is identical to the w ild-type a transcript for the entire coding region, but the 5'-untrans lated sequence of the a gene has been replaced by novel sequence. Here , we demonstrate that the novel 5' sequence in the A(y) mRNA correspon ds to the 5'-untranslated sequence of another gene that is normally ti ghtly linked to a in mouse chromosome 2. This other gene (Raly) has th e potential to encode a novel RNA-binding protein that is normally exp ressed in the preimplantation embryo, throughout development, and in a ll adult tissues examined. Importantly, the A(y) mutation disrupts the structure and expression of the Raly gene. The data suggest that the A(y) mutation arose from a DNA structural alteration that affects the expression of both agouti and Raly. We propose that the dominant pleio tropic effects associated with A(y) may result from the ectopic overex pression of the wild-type a gene product under the control of the Raly promoter and that the recessive embryonic lethality may be the result of the lack of Raly gene expression in the early embryo.