The goal of this project is to understand the functions of Drosophila RNA binding proteins, particularly those related to the vertebrate A/B group hnRNP proteins, a major class of nuclear proteins that bind premessenger RNA. Biochemical studies of vertebrate hnRNP proteins suggest that they are involved in packaging and splicing nascent transcripts. We are using genetic and molecular biology techniques to study five Drosophila proteins that have similar RNA binding domains, two of which, Hrb98DE and Hrb87F, are hnRNP proteins. A common feature of at least three of the proteins is that they are required for male fertility. Previously, we isolated a small deletion that removes the entire coding region of the Hrb87F gene. This mutation does not affect viability, but homozygous males are almost completely sterile. Recent experiments have shown that the deletion also removes a second gene, provisionally called BP1. The BP1 gene encodes a protein with two RNA binding domains that are similar to those of the Hrb proteins. The C-terminal domain is distinctive, and is unrelated to the other proteins. BP1 is expressed predominantly in pupae and adult males. Experiments are in progress to determine which gene is responsible for the male sterile phenotype and to analyze its function in spermatogenesis. Mutations in the Rb97D gene, another RNA binding protein, also lead to male sterility. Rb97D is expressed in the testes and several other tissues during development, but appears to be required only during spermatogenesis, since the only obvious phenotype of a null mutation is male sterility. EWS and TLS are human proteins that are disrupted in translocations asso- ciated with several sarcomas. A Drosophila homolog of these proteins is encoded by the p19 cDNA clone. The p19 protein has several glycine-rich regions, and a domain that weakly resembles the RNA binding domains of the Hrb proteins. We have demonstrated that a polypeptide consisting of just the putative RNA binding domain can be cross-linked to RNA in vitro using UV light. Genetic characterization of this locus and further biochemical analyses of the protein are underway.