Cell surface receptors for gibbon ape leukemia virus (Glvr-1) and marine amphotropic retrovirus (Ram-i) Ram-i (10) and human Glvr-1 (7) cDNAs were cloned into pGEM-7Z (Promega) with their 5' ends adjacent to the SP6 promoter. For mRNA synthesis, the plasmids were linearized and transcribed with SP6 polymerase in the presence of m7G(5')ppp(5')G caps according to the manufacturer's directions (Pharmacia). Xenopus laevis oocytes were injected with 50 nl of mRNA (1 ng/nl) or with an equal volume of H20 and were incubated for 4-6 days at 17'C; then two-microelectrode voltage-clamp recordings or radiolabel uptake assays were performed at room temperature as described (23).Abbreviations: GALV, gibbon ape leukemia virus; Glvr-1, cell surface receptor for GALV; Ram-i, cell surface receptor for amphotropic murine retrovirus; HIV, human immunodeficiency virus; MLV, murine leukemia virus; Mo-MLV, Moloney MLV.

Mutations in genes encoding ribosomal proteins cause the Minute phenotype in Drosophila and mice, and Diamond-Blackfan syndrome in humans. Here we report two mouse dark skin (Dsk) loci caused by mutations in Rps19 (ribosomal protein S19) and Rps20 (ribosomal protein S20). We identify a common pathophysiologic program in which p53 stabilization stimulates Kit ligand expression, and, consequently, epidermal melanocytosis via a paracrine mechanism. Accumulation of p53 also causes reduced body size and erythrocyte count. These results provide a mechanistic explanation for the diverse collection of phenotypes that accompany reduced dosage of genes encoding ribosomal proteins, and have implications for understanding normal human variation and human disease.

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