The Energy Cost of RNA Synthesis in Sea Urchin Embryos ( <i>Strongylocentrotus purpuratus</i> )

AbstractDefining the metabolic cost of specific biochemical processes is key to understanding strategies of energy (ATP) allocation. In this study, we determined the energy cost of RNA synthesis and the allocation of the ATP pool to this biosynthetic process. Rates of oxygen consumption, RNA synthesis, and protein synthesis were measured in early-stage embryos (single cell layer, 18-h-old blastula stage) of the sea urchin (<i>Strongylocentrotus purpuratus</i>) that have high rates of RNA synthesis. The inhibitory effects of actinomycin D on these processes were measured to calculate the energy cost of RNA synthesis. Across 15 different cohorts of embryos studied, the average rate of RNA synthesis was 0.19 ± 0.031 (SEM) ng RNA embryo^-1 h^-1. In the presence of actinomycin D, the rate of RNA synthesis decreased by 59%, and respiration decreased by 26%. As expected, actinomycin D also inhibited protein synthesis (by 28%) but had no effect on rates of uridine and alanine transport. This analysis revealed that the energy cost of RNA synthesis is 2.01 ± 0.24 (SEM) <i>μ</i>J (ng RNA synthesis)^-1-a value that is notably similar to the known cost of protein synthesis in this species of 2.40 ± 0.21 <i>μ</i>J (ng protein synthesis)^-1. In embryos, the rate of RNA synthesis was lower than that of protein synthesis, resulting in the former requiring only 11% of the total available ATP pool, compared to 67% for the latter. The significance of these findings is presented in the context of understanding the constraints and trade-offs of ATP allocation during development.