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Plant Physiology 61:20-24 (1978)
© 1978 American Society of Plant Biologists

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Articles

Leucine: tRNA Ligase from Cultured Cells of Nicotiana tabacum var. Xanthi

Evidence for de Novo Synthesis and for Loss of Functional Enzyme Molecules 1

Nigel R. Gore2 and John L. Wray3

Department of Biochemistry, University of St. Andrews, North Street, St. Andrews, Fife, KY16 9AL, Scotland

Leucine:tRNA ligase was assayed in extracts from cultured tobacco (Nicotiana tabacum) XD cells by measuring the initial rate of aminoacylation of transfer RNA with L-[4,5-3H]leucine. Transfer RNA was purified from tobacco XD cells after the method of Vanderhoef et al. (Phytochemistry 9: 2291-2304). The buoyant density of leucine:tRNA ligase from cells grown for 100 generations in 2.5 mM [15N]nitrate and 30% deuterium oxide was 1.3397. After transfer of cells into light medium (2.5 mM [14N]nitrate and 100% H2O) the ligase activity increased and the buoyant density decreased with time to 1.3174 at 72 hours after transfer. It was concluded that leucine:tRNA ligase molecules were synthesized de novo from light amino acids during the period of activity increase. The width at half-peak height of the enzyme distribution profiles following isopycnic equilibrium centrifugation in caesium chloride remained constant at all times after transfer into light medium providing evidence for the loss of preexisting functional ligase molecules. It was concluded that during the period of activity increase the cellular level of enzyme activity was determined by a balance between de novo synthesis and the loss of functional enzyme molecules due to either inactivation or degradation.

2 Present address: May and Baker Ltd., Ongar Research Station, Fyfield Road, Ongar, Essex CM5 OHW, England.

3 To whom requests for reprints should be addressed.

1 This research was supported by the award of a Science Research Council Studentship to N. R. G. and a Science Research Council Research Grant (B/SR/9566) to J. L. W.






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Copyright © 1978 by the American Society of Plant Biologists