Histone gene expression in the developing sea urchin embyro is regulated in a complex developmental program. Dramatic quantitative modulations in histone synthesis occur in concert with changes in the rate of embryonic DNA regulation, and are accompanied by an equally striking, stage-specific switch in the histone subtypes that are accumulated by the embryo. This switch in protein subtypes is caused by the sequential expression of two structurally distinct classes of genes whose transcripts differ in both size and sequence. The aim of this project is to contribute to the understanding of the molecular genetics of the sea urchin histone program; in particular, I propose to measure the rates of transcription, nuclear export, and decay in the cytoplasm of both early and late forms of embryonic histone mRNAs at different stages of sea urchin development. This approach will identify rate-limiting steps and will thus enable a detailed analysis of important regulatory processes. I will utilize a simplified, "reverse genetic" system to investigate the role played specific nucleotide sequences of cloned early and late histone genes in the regulation of histone gene expression. Early and late histone genes will be microinjected into sea urchin eggs and zygotes. The extent and fidelity of transcription will be assessed as a function of developmental time. In vitro mutagenesis will be used to identify specific sequences on the injected genes that are responsible for the stage-specific regulation. Preliminary evidence suggests that the half-life of early histone mRNAs decreases at a particular developmental stage. This decrease, apparently specific to the early histone mRNAs, results in the rapid disappearance of this class of mRNAs. I intend to study the mechanism of the change in half-life through the use of a cell free system. I will prepare extracts of embryos at stages before, during and after the half-life change. I will then determine the decay kinetics of the early histone mRNAs in these fractions. If the decay rate of early histone mRNAs is found to undergo the expected, stage-specific alteration in relation to other cellular mRNAs, I will attempt to isolate factor(s) from cell embryo extracts that are involved in the half-life change.