RIBOSOMAL-PROTEIN INSUFFICIENCY AND THE MINUTE SYNDROME IN DROSOPHILA- A DOSE-RESPONSE RELATIONSHIP

Minutes comprise >50 phenotypically similar scattered throughout the g enome of Drosophilia, many of which are identified as mutations in rib osomal protein (rp) genes. Common traits of the Minute phenotype are s hort and thin bristles, slow development, adn recessive lethality. By mobilizing a P element inserted in the 5' UTR of M(3)95A, the gene enc oding ribosomal protein S3 (RPS3), we have generated two homozygous vi able heteroalleles that are partial revertants with respect to the Min ute phenotype. Molecular characterization revealed both alleles to be imprecise excisions, leaving 40 and 110 bp, respectively, at the P-ele ment insertion site. The weaker allele (40 bp insert) is associated wi th a similar to 15% decrease in RPS3 mRNA abundance and displays a mod erate Minute phenotype. In the stronger allele (110 bp insert) RPS3 mR NA levels are reduced by similar to 60%, resulting in an extreme Minut e phenotype that includes many morphological abnormalities as well as sterility in both males and females due to disruption of early gametog enesis. The results shoe that there is a correlation between reduced R PS3 mRNA levels and the severity of the Minute phenotype, in which fau lty differentiation of somatic tissues and arrest of gametogenesis. Th e results show that there is a correlation between reduced RPS3 mRNA l evels and the severity of the Minute phenotype, in which faulty differ entiation of somatic tissues and arrest of gametogenesis represent the extreme case. That heteroalleles in M(3)95A can mimic the phenotypic variations that exist between different Minute/rp-gene mutations stron gly suggests that all phenotypes primarily are caused by reductions in maximum protein synthesis rates, but that the sensitivity for reduced levels of the individual rp-gene products is different.