CONTROL OF GLUTAMINE SYNTHETASE IN THE EMBRYONIC RETINA IN VITRO.

Glutamine synthetase activity in the neural retina of the chick embryo increases sharply during terminal differentiation of this tissue. This characteristic increase can be reproduced in cultures of retinal tissue fragments from late embryos. A similarly sharp increase can be elicited precociously in younger retina by culturing in medium with adult serum. Both the precociously elicited and the later increase in enzyme activity require continuous protein synthesis; both can be suppressed during the first 24 hours of culture by blocking RNA synthesis or by removing the adult serum. Subsequently, the increase in enzyme activity becomes progressively less dependent on RNA synthesis and on the continuous presence of adult serum. This transition is attained more rapidly in the older retina. The data suggest a progressive stabilization of the enzyme-forming system during differentiation.