A role for asparaginyl-tRNA in the regulation of asparagine synthetase in a mammalian cell line.

The expression of asparagine synthetase activity [L-aspartate:ammonia ligase (AMP-forming), EC 6.3.1.1] in cultured Chinese hamster ovary (CHO) cells is regulated by asparagine. After transfer of CHO cells from an asparagine-supplemented medium to a medium lacking asparagine, activity increases 1.5- to 2-fold. If asparagine is added back to the medium, activity returns to control levels. To test the possible involvement of Asn-tRNAAsn in regulating the levels of asparagine synthetase, we have examined the levels of asparagine synthetase in a mutant of CHO cells containing a temperature-sensitive asparaginyl-tRNA synthetase [L-asparagine:tRNA ligase (AMP-forming), EC 6.1.1.22]. Under conditions of limited asparaginyl-tRNA synthetase activity in the mutant, there is a 2- to 3-fold increase in the level of asparagine synthetase activity. Under identical conditions, there is no change in asparagine synthetase activity in the wild type. This correlation between asparaginyl-tRNA synthetase activity and asparagine synthetase levels may be a consequence of a direct role of tRNAAsn in the regulation of the in vivo expression of the asparagine synthetase structural gene.