Characterization of leucyl-tRNA synthetase complexes in CHO-human hybrid cell lines and tsHl transport revertants
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The leucyl-tRNA-synthetase (LeuRS) complexes from three Chinese hamster ovary (CHO) revertants were characterized by sucrose gradient sedimentation analysis. The mutant lines D-10, F-10 and C-ll are temperature resistant revertants of the temperature sensitive LeuRS mutant tsHl and have larger intracellular pools of leucine and other L System amino acids. Thus, the reversion of their temperature sensitive phenotype may be due to a revertant LeuRS or alterations in the regulation of transport System L. The revertants D-10 and C-ll showed the enzyme profile and thermolability of the mutant enzyme from tsHl indicating they may be transport revertants. However, F-10 may be a partial enzyme revertant due to the increased thermolability of its LeuRS and slight restoration of the high molecular weight complexes. A heat resistant hybrid cell line from fused ts025Q and human leukocytes, used to map the human gene for LeuRS, was also studied along with three hybrid subclones. These were used to distinguish human and hamster LeuRS in the hybrid. The characterization confirms that it is indeed the structural gene for human LeuRS being mapped and not a protein component that stabilizes the heat sensitive hamster LeuRS in the hybrid. A high molecular weight complex is present in the hybrid and not in ts025Cl or human lines. The formation of the high molecular weight complex in the hybrid remains to be completely elucidated. The characterization of LeuRS from the leucyl-tRNA synthetase mutant ts025Cl showed that it was uniquely altered in the high molecular weight complexes. The low molecular weight form maintained the thermolability and K[sub m] of the low molecular weight form from wild type CHO cells indicating no alteration of the catalytic site.