Faster protein degradation in response to decreases steady state levels of amino acylation of tRNAHis in Chinese hamster ovary cells.

The rate of protein degradation in cultured Chinese hamster ovary cells increases in response to histidine starvation. Using cell lines with defective histidyl-tRNA synthetase, or histidinol (a competitive inhibitor of the enzyme), we have previously demonstrated a functional connection between the increase in degradation and the amino acylation of this tRNA (Scornik, O. A., Ledbetter, M. L. S., and Malter, J. S. (1980) J. Biol. Chem. 255, 6322-6329). A correlation is shown here between the steady state level of histidyl-tRNA and the regulatory response. Cells were incubated for 15 min in the presence of L-[3H]histidine, at a concentration at which greater than 90% of histidine for protein synthesis derives from the medium. The level of histidyl-tRNA was measured by its radioactivity after purification by phenol extraction, ethanol precipitation, and mild alkaline hydrolysis. Protein degradation in each condition was determined by the release of acid-soluble radioactivity from cells labeled for 24 h with L-[1-14C]leucine. The steady state level of histidyl-tRNA was altered by either histidinol (which slows down its production) or cycloheximide (which interferes with its utilization). Cycloheximide counteracts the effects of histidinol both on the level of histidyl-tRNA and on the rate of protein degradation. Both effects can be obtained, however, even in the presence of cycloheximide, if higher concentrations of histidinol are used. The results indicate that this regulatory mechanism does not recognize the rate of amino acylation per se but rather, the steady state level of its product, amino acyl-tRNA.