Repression of arginase synthesis in Chang liver cells

Abstract

A rapid increase in specific arginase activity occurred in Chang liver cells immediately after addition of the missing nutrient to cells preincubated in a medium from which one essential amino acid had been omitted. The rapid change in enzymic activity offered an opportunity for studying specific enzyme regulation during short time experiments. 1. 1. The experimental results indicate that the increase in specific arginase activity, which could be immediately stopped by the addition of puromycin, was due to a synthesis of the enzyme more rapid than in normally growing cells. 2. 2. Actinomycin D at concentrations sufficient to inhibit DNA directed RNA synthesis did not prevent the increase in arginase activity, suggesting that the synthesis of the enzyme took place in response to a preformed stable messenger RNA. 3. 3. A few hours after the addition of the missing nutrient to a prestarved culture, arginase synthesis was again retarded. 3.1. (a) This decrease in the rate of enzyme formation could be prevented by addition of actinomycin D. The decrease in the rate of enzyme synthesis in absence of actinomycin, therefore, might not be due to a destruction of the specific messenger RNA, but rather to a depression of the rate at which the stable arginase messenger was translated. 3.2. (b) When proline was added to the culture medium the decrease in the rate of enzyme formation was enhanced. The experimental results suggest a regulation of arginase synthesis on the level of translation by means of an actinomycin sensitive repressor. The activity of this repressor seems to depend on the presence of a low molecular co-repressor, which accumulates in the cells in presence of proline. 4. 4. When proline was added to the deficient medium during the starvation period, the increase in arginase activity immediately after the addition of the missing nutrient was partially prevented. 4.1. (a) An increased rate of arginase formation was produced when these cells, prestarved in presence of proline, were transferred to a “normal” growth medium without added proline. 4.2. (b) This stimulation of arginase synthesis, brought about by the removal of proline, was not prevented by the addition of actinomycin D and therefore seemed not to be dependent of a formation of new messenger RNA. These results again suggest a proline dependent repression of arginase synthesis at the level of translation of a preformed, stable messenger RNA.