Transcriptional and Translational Inducers

The use of large Petri dishes for the culture of chick embyro liver cells has made possible the direct analysis of ALA-synthetase [21]. With this procedure, it has been determined that there are two controls 

The induction is specific, not general. The increase in ALA-synthetase induced by AlA or DDC is not due to an overall rate of synthesis of mRNA or protein, for neither radioactive orotic acid nor uridine, nor radioactive leucine was incorporated at a greater rate in the presence than in the absence of the inducers [25, 128]. 

The enzyme itself is not affected by inducing agents. The chemicals did not increase or decrease the activity or lifetime of ALA-synthetase, whether in isolated mitochondria or in cells in culture. In the simultaneous presence of acetoxycycloheximide (to block protein synthesis) and an inducing chemical, the decay rate of ALA-synthetase was identical with that of the control without inducing chemical—that is, ti/2 = 3 hour (Table 111). When the inducer was given for 14 hours to 

Actions of Inhibitors and Inducers on ALA-Synthetase Formation and Decay  

Overall half-life for ALA-synthetase. After addition of actinomycin D, ALA-synthetase increased slightly for 2 hours then the activity remained at a plateau for 3 hours, and finally it decayed with ti/2 of 3 hours. In actinomycin D -h heme treatment, the decay of ALA-synthetase took place within 3 hours of drug addition. 

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