REGULATION OF THE INITIATION OF HEMOGLOBIN SYNTHESIS IN THE BLOOD ISLAND CELLS OF CHICK EMBRYOS: I. QUALITATIVE STUDIES OF THE EFFECTS OF ACTINOMYCIN AND δ-AMINOLEVULINIC ACID

Abstract

Intact and de-embryonated blastodiscs of chick embryos from all stages of development between the definitive primitive streak and the 10-somite stage were incubated on simple solid synthetic media. On the basal medium, blastodiscs at all initial stages of development synthesized hemoglobin readily visible to the naked eye within 24 hours, incorporated leucine into protein at an approximately linear rate for 24 hours, and incorporated uridine into RNA at a roughly linear rate for at least 6 hours after a short lag.Blastodiscs taken before the 1-somite stage failed to synthesize any detectable hemoglobin on medium containing 2 μg/ml of actinomycin, whereas those token at later stages synthesized hemoglobin visible to the naked eye. This concentration of actinomycin totally inhibited the incorporation of uridine into high molecular weight RNA within 2–3 hours, but the incorporation of leucine into protein was not inhibited for 6–8 hours. The residual incorporation of uridine was entirely into the soluble RNA fraction.At 10 μg/ml, actinomycin markedly inhibited the synthesis of hemoglobin by blastodiscs taken at stages earlier than the 6-somite embryo, but did not markedly affect hemoglobin synthesis by the more advanced blastodiscs. This concentration of actinomycin caused only slightly greater inhibition of the incorporation of uridine into acid-precipitable material than the smaller concentration for all blastodiscs, and was not markedly more inhibitory for the incorporation of leucine into protein.The presence of δ-aminolevulinic acid overcame the inhibitions of synthesis of hemoglobin by actinomycin but did not prevent the inhibitions of incorporation of uridine into RNA and of leucine into protein.Regulation of the onset of rapid hemoglobin synthesis appears to be at the translation level, probably through the supply of δ-aminolevulinic acid. The latter is probably regulated through synthesis of RNAs formed at the head-fold stage. Messenger RNAs for globin synthesis are present at the stage of the definitive primitive streak.