Multiple levels of posttranscriptional control lead to germ line-specific gene expression in the zebrafish.

An important mechanism for the specification and development of the animal germ line is the localization of specific molecules to the germ plasm. Restriction of these molecules to the germ line is considered to be critical for proper development of the germ line as well as the soma. Cytoplasmic localization alone, however, may not be sufficient to achieve germ line-specific expression. While zebrafish vasa mRNA is localized to the germ plasm, the Vasa protein is initially distributed uniformly in the embryo, and its expression becomes restricted to the PGCs only later in development. Here, we demonstrate that, in addition to vasa RNA localization, multiple cell type-specific posttranscriptional mechanisms act on vasa mRNA and Vasa protein. We show that the portion of the maternal vasa mRNA, which is partitioned to somatic cells, is rapidly degraded, whereas vasa RNA is stabilized in the PGCs in a process that is mediated by cis-acting elements within the molecule. Similarly, the Vasa protein is highly unstable in somatic cells, but not in the PGCs. Finally, we demonstrate that subcellular localization of Vasa protein involves cis-acting domains within the protein. In conclusion, we show that posttranscriptional degradation-protection mechanisms acting on RNA and protein function in a vertebrate to enrich for specific molecules in the PGCs.