Control of Glutamine Synthetase Synthesis in the Embryonic Chick Neural Retina

The cortisol induction of the enzyme glutamine synthetase (EC 6.4.1.2) in the embryonic chick neural retina is controlled initially at the transcriptional level, as previously demonstrated by the unique appearance of a 15 S RNA species which codes for a single and identical polypeptide chain of glutamine synthetase. This is found only on polyribosomes of cortisol-treated retinas. Recent experiments based on the apparent paradoxical effects of actinomycin D have suggested that glutamine synthetase synthesis may also be regulated translationally. It was proposed that a translational repressor and degrader of glutamine synthetase messenger existed with properties which allowed it to be synthesized at low concentrations of actinomycin D, but blocked at high actinomycin D levels. A derepressor molecule was postulated which is induced by the addition of cortisol to the retina. It purportedly acts to nullify the repressor and is blocked at low actinomycin D concentrations (Moscona, A. A., Moscona, M. H., and Saenz, N. (1968) Proc. Nat. Acad. Sci. U. S. A. 61, 160). Actinomycin D in high concentrations in culture with cortisol-induced retina allowed glutamine synthetase to be rapidly synthesized only for 4 hours, while low doses allowed glutamine synthetase synthesis to continue for over 20 hours at a slow but continuous rate. Low concentrations of actinomycin D did not selectively repress glutamine synthetase synthesis, in comparison with other supernatant proteins, as would be suggestive of specific translational control. However, the treatment caused an over-all reduction in protein synthesis. A translational theory cannot be supported in the face of data presented here of deleterious effects of actinomycin D on oxidative respiration, the ATP pool size, the polyribosome content, and the histological appearance of the retina. The disappearance of polyribosomes and the severe decrease in protein synthesis after actinomycin D treatment is considered to be a poor index of messenger RNA stability, since undegraded messengers inhibited in translation were found in actinomycin D-treated retinas. Actinomycin D not only acts as an inhibitor of RNA synthesis, but also as a nonspecific metabolic poison.