Giant polytene chromosomes offer unique genetic and cytological advantages for studying the control of transcription in multicellular eukaryotes. A method for the isolation of gram quantities of salivary glands from Drosophila hydei will be used to exploit this material for biochemical studies of the regulation of RNA metabolism. Four problems will be investigated with this system. 1. As yet there is no difinitive demonstration that a given puff is responsible for a given cellular protein. The genetic locus for the structural gene of a salivary gland DNase will be mapped with the aid of available electrophoretic variants. This information will be used in an attempt to relate the appearance of a chromosomal puff with the appearance of the enzyme in the tissue. 2. A large accumulation of acidic protein is observed at polytene chromosome bands immediately prior to puffing and RNA synthesis. This observation suggests that the protein is involved in the earliest stages of gene activation. Attempts are being made to isolate and characterize this protein, in order to provide an understanding of its function. 3. Molecular hybridization studies are being performed with RNA recovered from salivary glands that exhibit different puffing patterns. 4. Preliminary studies on the effect of BUdR on chromosomal puffing are being completed. Attempts will be made to select radiation sensitive mutants of Drosophila melanogaster. Mutants will be characterized for their ability to perform repair replication.