Regulation of Protein Synthesis in Chick Oviduct: III. MECHANISM OF OVALBUMIN “SUPERINDUCTION” BY ACTINOMYCIN D

Abstract

Abstract Superinduction is a phenomenon frequently observed in eukaryotic cells, whereby the inhibition of RNA synthesis (usually with actinomycin D) results in an increase in the concentration (activity) of a specific protein. This paper shows that actinomycin D can superinduce the major egg white proteins synthesized in chick magnum. After actinomycin D treatment, ovalbumin, conalbumin, ovomucoid, and lysozyme became an increased percentage of the total protein synthesized regardless of whether (a) the drug was administered to chicks or added to magnum explants in culture; and (b) protein synthesis was measured in vivo or in culture. Whereas the rate of nonsecretory protein synthesis declined with a half-life of about 5 hours after actinomycin D treatment of magnum explants, the rates of ovalbumin and conalbumin synthesis declined with half-lives of 14 and 8 hours, respectively. Furthermore, actinomycin D increased the absolute rate of ovalbumin and conalbumin synthesis compared to untreated controls, despite a small decrease in the number of polysomes and the concentration of total ovalbumin mRNA. Polypeptide elongation normally limits the rate of protein synthesis in the estrogen-stimulated magnum. Two independent methods reveal that actinomycin D treatment increased the rate of elongation 10 to 40%. The average polysome size, and the size of ovalbumin-synthesizing polysomes in particular, was maintained when actinomycin D was administered to estrogen-stimulated chicks indicating that there was a corresponding increase in the rate of initiation. During hormonal withdrawal, when initiation limits the rate of protein synthesis, actinomycin D treatment resulted in larger polysomes and an increased rate of elongation compared to untreated controls, suggesting that actinomycin D enhanced the rate of polypeptide initiation more than elongation. We propose a model to explain superinduction in this system which is based on (a) differential stability of mRNAs; and (b) competition of mRNAs for rate-limiting factors common to the translation of all mRNAs. Hence, after inhibiting RNA synthesis, the long lived mRNAs increase in proportion to the total mRNA population remaining, and can be translated at an increased rate because of more favorable competition for the rate-limiting factors.