Activity of aminoacyl-transfer-ribonucleic acid synthetases in tobacco-leaf tissue in relation to senescence and to the action of 6-furfurylaminopurine.

1. The absolute aminoacyl-transfer-RNA synthetase activity decreased with increasing physiological age of the tobacco leaf, but the specific activity did not decrease until the chlorophyll content fell below about 20% of that in the young leaf. 2. In leaf disks floated on water, the absolute aminoacyl-transfer-RNA synthetase activity increased after 2 days but then decreased until the end of the experiment (12 days). Since the concentration of soluble protein decreased, the specific activity increased throughout the experiment. 3. The absolute and specific aminoacyl-transfer-RNA synthetase activities increased above control values in disks treated with 6-furfurylaminopurine (kinetin) after 4 and 6 days respectively, but the effect was too slow to account for the delay in decrease of chlorophyll and the delay in increase of alpha-amino nitrogen induced by kinetin. 4. After treatment for 7 days, the absolute aminoacyl-transfer-RNA synthetase activity increased in disks treated with kinetin between 0.5 and 1.6mum but the concentration of alpha-amino nitrogen decreased with kinetin concentrations between 0.05 and 50mum.