Spatial and temporal control of RNA stability

Maternally encoded RNAs and proteins program the early development of all a nimals. A subset of the maternal transcripts is eliminated from the embryo before the midblastula transition. In certain cases, transcripts are protec ted from degradation in a subregion of the embryonic cytoplasm. thus result ing in transcript localization. Maternal factors are sufficient for both th e degradation and protection components of transcript localization. Cis-act ing elements in the RNAs convert transcripts progressively (i) from inheren tly stable to unstable and (ii) from uniformly degraded to locally protecte d. Similar mechanisms are likely to act later in development to restrict ce rtain classes of transcripts to particular cell types within somatic cell l ineages. Functions of transcript degradation and protection are discussed.