The dynamics of maternal poly(A)-containing mRNA in fertilized sea urchin eggs

Cytoplasmic polyadenylated RNA with the characteristics of sequestered mRNA exists in the unfertilized sea urchin egg. Following egg activation, the amount of poly(A) doubles, but total RNA content stays constant. Chromatography of the RNA on poly(U)-Sepharose shows that the amount of RNA that bears a poly(A) tract increases slightly (approximately 20-30%) during the 2 hr after fertilization. When a cDNA transcript of the poly(A)+ mRNA from 2 hr zygotes is reacted against poly(A)+ RNA from either eggs or zygotes, the kinetics of reassociation of the two preparations seem identical; hence the RNA sequences bearing poly(A) are the same in eggs and zygotes. Measurement of the length of the poly(A) tract in eggs and zygotes shows an increase in number average length from about 45 bases to 60 bases. Measurement of tract length of poly(A) in two cell zygotes by adenosine/AMP ratios of radioactive RNA shows that the poly(A) tract of the zygote is solely accounted for radioactive RNA, indicating extensive turnover of the poly(A). It is concluded that the poly(A) tract in these cells is subject to both lengthening and shortening, with the former predominating in this instance. the increase in poly(A) does not involve polyadenylation of different sequences, but is due to an increase in the number of polyadenylated sequences and the length of the poly(A) tracts that they bear.