We propose a genetic analysis of tRNA structure and function including work on synthesis and function of modified bases, and the mechanims of tRNA's role in protein synthesis and in gene regulation. Three main lines of work will be undertaken: 1. Analysis of suppressor mutations which affect tRNA sequence and base modification. Projects will include analysis of codon context effects on tRNA efficiency, new UGA suppressors and genetic analysis of the biosynthetic pathway for a modified base, the methyl ester of uridine-5-oxyacetic acid. 2. Analysis of the regulatory mechanism for the histidine operon. We will determine the DNA sequence changes for more mutants in the control (his0) region in order to refine the current model for tRNA's involvement in this mechanism. This will involve study of the derepressive effect of rif(r) and str(r) metations on operon expression. We also hope to learn how the high basal levels of operon expression are set. We will try to determine the mechansim where by the operon is subject to metabolic (ppGpp) control. The possibility of regulatory interactions between the control mechanisms for the histidine and purine pathways will be investigated. 3. Analysis of the regulatory mechanism for proline degradation. We will determine whether or not tRNA(PRO) is involved in regulation of this system and how the degradative enzyme (a bifunctional oxidase-dehydrogenase) manages to act in a regulatory capacity and also be a bifunctional, membrane-associated enzyme.