A maternal tail of poly(a): The long and the short of it

Rosemary F. Bachvarova Department of Cell Biology Cornell University Medical College New York, New York 10021 Just as most cells transcribe only some of their many genes, so growing oocytes, maturing oocytes, and early embryos translate only some of the many messages pres- ent in the cytoplasm. A large fraction of the mRNA synthe- sized in growing oocytes is not used for immediate transla- tion, but is dormant or masked, and stored for future translation. Most stored messages are activated at various times during oocyte maturation and early embryonic de- velopment, when little or no mRNA is produced. In general, stored messages carry short poly(A) tails (15-90 A’s), and activation is accompanied by cytoplasmic polyadenylation increasing the tail to 150 A’s or more. Likewise, cessation of translation is accompanied by deadenylation of the mes- sage. The poly(A) tail is known to promote initiation of translation (reviewed in Jackson and Standart, 1990) and in yeast, thiseffect is mediated through the poly(A)-binding protein, whose gene is required for viability. Two classes of maternal message whose translation is regulated during meiotic maturation have been studied in frog and mouse oocytes (reviewed in Wickens, 1990; Richter, 1991). First is a set of stored messages whose translation commences during meiotic maturation (see tPA mRNA in Figure 1). The several members studied are all polyadenylated in a process that requires specific sequences in the 3’ untranslated region (UTR): the poly-