RNA synthesis in the salivary glands of Chironomus tentans decreased to about 5% of controls within less than 1 h in larvae treated with actinomycin. In older intermolt larvae glandular protein synthesis continued at a similar rate as before addition of the drug even after RNA synthesis had been inhibited for 24, and for 48 h. Increases in protein synthesis during the experiment were blocked by actinomycin. However, the enhanced rates of protein synthesis again were maintained for at least 24 h, if RNA synthesis was inhibited subsequently. The incorporation of amino acids was completely blocked by inhibitors of protein synthesis, such as cycloheximide and puromycin. During the first day of the last larval instar (the RH-stage) the rate of amino acid incorporation increased about 1 1 2- to 2-fold. The rate of protein synthesis in RH-larvae which had been treated for 24 h with actinomycin was about 50% of the rate in timed controls, and 60 to 80% of the rate before application of the drug. The degree of inhibition was positively correlated with the relative rate of amino acid incorporation. RH-larvae aged for 6 h showed a higher degree of inhibition than non-aged RH-larvae. The actinomycin-sensitivity of protein synthesis, thus, seems to change during the early stages of the last instar. The rate of protein synthesis did not decrease beyond its level at 24 h in actinomycin, if the treatment was prolonged to 48 h. In 24 or 48 h old larvae protein synthesis was no longer sensitive to actinomycin. Injection of ecdysone into older intermolt larvae increased the rate of glandular protein synthesis, and this effect was prevented by actinomycin. In view of the large variation encountered, no attempts were made to determine the dependence of this protein synthesis on continued RNA synthesis. The results are interpreted in terms of mRNA-stability. Accordingly, it is concluded that glandular protein synthesis is essentially supported by stable mRNA during the last instar. At its beginning however, there is a transient period where protein synthesis is supported by unstable RNA. This pattern of changing stability of RNA is discussed with regard to chromosomal puff activity and to the developmental programming of the cells. © 1969.
