Developmental changes in the synthesis of sea urchin embryo messenger RNA containing and lacking polyadenylic acid

Abstract

The synthesis of three classes of messenger RNA has been studied as a function of the development of the sea urchin embryo. These classes are non-histone mRNA containing poly(A), “[+A]mRNA”; nonhistone mRNA lacking poly(A), “[−A]mRNA”; and putative histone mRNA, which lacks poly(A). The pulse-labeled RNA distributed among these classes is shown to be associated with polyribosomes, by virtue of its banding with formaldehyde-fixed polyribosomes in CsCl gradients and its dissociation from polyribosomes by EDTA treatment. Extensive changes in transcription occur at successive embryonic stages. During the rapidly replicating late cleavage and early blastula stages, histone mRNA synthesis is predominant, accounting for 60% of the labeled RNA in free polyribosomes. At this time [+A]mRNA accounts for approximately 10% of the RNA label. During development from the mid- to late blastula, the relative amount of histone mRNA synthesized decreases sharply, and that of [+A]mRNA increases to approximately 50% of labeled polysomal RNA at the mesenchyme blastula stage. Thereafter, in development to the late gastrula stages, the proportion of [+A]mRNA synthesized increases slightly and that of histone mRNA synthesized decreases slightly. During this whole course of development, the proportion of nonhistone [−A]mRNA synthesized remains essentially constant at approximately 30% of the labeled polysomal RNA. Thus, the syntheses of these three mRNA classes appear to be under separate controls. The average molecular weight of mRNA in small polyribosomes is considerably larger than that which can be fully loaded by the number of ribosomes present. Hence a certain fraction of the mRNA is not fully loaded with ribosomes, and the degree of loading is indicated by the ratio of mRNA in large compared to small polyribosomes. This ratio for the [+A]mRNA in the mid-blastula indicates underloading; however, the ratio increases with development to the late gastrula, indicating an approach to maximal ribosomal loading. During the same period [−A]mRNA is apparently under-loaded, and there is little or no change in its degree of loading. Either there are large stretches of untranslated nucleotide sequences, which are thereby not occupied by ribosomes, or else the [−A]mRNAs persist as inefficiently loaded. We propose that in the late stage embryos, there is an increase in initiation activity which promotes ribosomal binding to [+A]mRNA more effectively than to [−A]mRNA.