Isolation of Chinese hamster ovary cell mutants defective in the regulation of leucine transport.

We have isolated mutants defective in the regulation of leucine transport by selecting temperature-resistant revertants from the CHO-tsH1 strain, a temperature-sensitive leucyl-tRNA synthetase mutant of the Chinese hamster ovary cell line. In each revertant, there is a stable 2- to 3-fold enhancement in the activity of transport System L, which serves for the uptake of branched-chain amino acids. This increased transport is reflected by an increase in the Vmax for System L transport without a significant change in the Km value and results in increased intracellular pools of leucine. The thermal stability of the leucyl-tRNA synthetase activity in each of these revertants is not changed significantly from that of the starting strain, CHO-tsH1. We conclude from these studies that the temperature resistance in the revertants arises from alterations in the regulation of transport System L, leading to constitutively high System L transport and increased intracellular pools of leucine, complementing the leucyl-tRNA synthetase defect.