Two temporal phases for the control of histone gene activity in cleaving sea urchin embryos (S. purpuratus)

This report presents an analysis of histone gene expression in the cleaving embryo of the sea urchin, Strongylocentrotus purpuratus, with emphasis on whether the regulatory site(s) in the pathway of gene expression change as development proceeds. The analysis focuses on the equation, $\text{dP1}\text{dt}\text{= M·f·n·}\text{A}\text{t}$, where $\text{dP1}\text{dt}$ = the absolute rate of histone synthesis; M = the mole quantity of histone messenger RNA; f = the fraction of histone mRNA in polysomes; n = the polysome size; and $\text{A}\text{t}$ = the rate of elongation of nascent histone polypeptide chains. The embryo solves this rate equation differently at different times. Measurements were made (at 15°C) of absolute rates of histone synthesis ($\text{dP1}\text{dt}$). The rate of histone synthesis increases at least 48-fold during the first 6 hr after fertilization from less than 0.5 to 24 pg embryo^?1 hr^?1; in the period from 6 to 12 hr, this rate rises to 182 pg embryo^?1 hr^?1, an additional 7.7-fold rise, resulting in an overall increase of 370-fold between the 1-cell and 200-cell stage. The fraction of newly synthesized (zygotic) histone messenger RNA that partitions into polysomes (f^zygotic) has also been measured during the first 12 hr of development. This fraction increases from 0.2 in the 2-hr embryo to 0.8 in the 6-hr embryo (16-cell stage), increasing slowly thereafter to near unity by 12 hr. The size of histone-synthesizing polysomes (n) does not change substantially over the 12-hr interval, remaining constant at a weighted mean of 5 ribosomes per polysome (range 3 to 7). Utilizing the data on f^zygotic and $\text{dP1}\text{dt}$, the rate of elongation of nascent histone polypeptide chains ($\text{A}\text{t}$) during the first 6 hr of development was estimated; $\text{A}\text{t}$ remains constant at 1.11 codons per second. This calculated value is in fair agreement with a direct measurement of histone peptide elongation rate in the 12-hr embryo. It is proposed that histone gene expression in cleaving sea urchin embryos be divided into two phases, distinguished on the basis of their pivotal translational parameters: Phase I (0–6 hr), during which f is rate determining, and Phase II (6 hr on), during which M is the rate-determining parameter.