This project is directed towards an understanding of the rules which govern the expression of genes, particularly those rules concerned with the formation of mature messenger RNAs from primary transcripts. One area of emphasis is on the mechanism by which transposable element insertions within the transcribed regions of genes affect the production of messenger RNAs from those genes. The importance of this problem is underscored by the observation that insertion mutations account for the majority of spontaneous mutations in the fruit fly Drosophila and are responsible for at least some human genetic disease. The mutant allele white-apricot has been chosen as the focus for this study because of the relative ease with which second-site modifiers of the expression of white-apricot can be obtained and studied. The product of this research on white-apricot, attainable within the five year period, should be a mechanistic description at the molecular level of the suppression and enhancement of white-apricot by second-site modifiers. Current indications are that modifiers of white-apricot are genes that encode proteins with roles in the processing of messenger RNAS. Specific sequences that are essential for the response of white-apricot to suppressors and enhancers will be identified by testing the effects of mutations in these sequences. This will be done by analyzing phenotype (eye color and RNA products) in transformed flies as a function of genetic modifiers, and by in vitro splicing experiments. In addition, we will continue to identify and characterize modifier loci by genetic analysis and molecular cloning. A second, related, area of emphasis is the Ul small nuclear ribonucleoprotein particle (snRNP), a component of the higher eukaryotic splicing apparatus which functions in the selection of splice sites at the 5' ends of introns. We will carry out a genetic analysis of components of the Ul snRNP. One protein of the UlsnRNP, the so-called 70K protein, contains a region of sequence similar to portions of three genetically identified splicing regulators in Drosophila, including suppressor-of-white-apricot. Specific hypotheses for the function of this domain will be tested by transforming a strain bearing a mutation in the wild type 70K gene with altered forms of this gene, and by complementary biochemical analysis.