Most cells respond to amino acid deprivation with a number of compensatory or adaptive biochemical changes including slower synthesis of protein and RNA, stimulated protein breakdown and growth arrest. In bacteria this "stringent response" is mediated by ppGpp, a regulatory nucleotide formed when deacylated tRNA binds to idle ribosomes. Mammalian cells respond to amino acid deprivation with analogous metabolic changes but ppGpp has not been found and the regulatory role of tRNA has been questioned. Work from this laboratory has provided clear evidence that in the stimulation of protein degradation by histidine starvation in CHO cells, the regulatory mechanism does recognize the level of his- tRNA and ribosomes idling in the histidine codon. Therefore, a mechanism does exist in mammalian cells comparable to the bacterial stringent response, except for the absence of ppGpp. This research will: (i) inquire into the nature of the signal emerging from idle ribosomes and attempt to reproduce some of the regulatory events in cell-free systems; and (ii) explore the physiological significance of this tRNA-dependent response in growth control, protection from mistranslation, and maintenance of amino acid pools. Unicellular organisms are exposed to changes in the environment in which they live. They compensate for the scarcity of some substances by increased transport or biosynthesis and adjust their growth rate to the availability of nutrients. In higher organisms, tissue specialization and hormonal regulation maintain relatively constant concentrations of nutrients in the body fluids. Mammalian cells are capable, however, of a direct response to amino acid deprivation which includes slower synthesis of RNA and protein, faster protein breakdown, and arrest of growth. The response is analogous to that in bacteria where the physiological changes following deprivation of a required amino acid are collectively known as the "stringent response". The bacterial stringent response is the result of a regulatory mechanism sensitive to the level of amino acylation of tRNAs. Evidence from this laboratory supports the existence of a comparable mechanism in cultured cells.
