Synthesis, turnover, and stability of heterogeneous RNA in growing mouse oocytes

Abstract

RNA of midgrowth phase naked mouse oocytes was labeled for 1 to 48 hr in vitro with [ 3H]uridine and displayed on 2% polyacrylamide gels. From the gel profiles and the total incorporation into RNA at each time point, the cumulative incorporation into hRNA >36 S, hRNA ≤36 S, and rRNA was computed. Assuming that rRNA is stable and that the rate of synthesis of rRNA is about 0.4 ng (the amount found in the egg) per 2 weeks (length of the oocyte growth phase), the incorporation data were converted to mass. The steady-state level of hRNA >36 S was 15 pg/nucleus, while hRNA ≤36 S (after some initial turnover) accumulated linearly at a rate of 0.01 pg/min. Correcting for the changing specific activity of the pool, reflected in the changing rate of incorporation into rRNA, the t 1 2 of hRNA >36 S was found to be less than 25 min. Assuming a t 1 2 of 20 min, the rate of synthesis of hRNA was 0.6 pg/min, a rate higher than that in mouse L cells, but 40X lower than expected for lampbrush chromosomes with a high rate of transcriptional activity. The available evidence indicates that very active lampbrush chromosomes do not exist at any stage in meiotic prophase of mouse oocytes.