Pulse-labeled ribonucleic acid (RNA) was extracted from polysomes of sporulating cells of Saccharomyces cerevisiae and characterized in sucrose gradients and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Transfer RNA, ribosomal RNA, and heterodisperse RNA, presumed to be messenger RNA, were synthesized during a 20-min pulse at T4 and T6 when labeling was performed in sporulation medium adjusted to pH 6.0. Furthermore, ribosomal RNA was processed into functional ribosomes during the pulse. The specific activity of pulse-labeled RNA of cells labeled in sporulation medium where the pH was unadjusted at T4 (pH 7.8) and T9 (pH 8.6) was 20- to 50-fold lower than RNA from cells labeled at pH 6.0. The low specific activity resulted from a 50-fold reduction in uptake of labeled precursors when the medium pH was greater than 7.2. However, heterodisperse RNA ranging from 4-17S in size and transfer RNA were synthesized during the pulse at T4 (pH 7.8),but the low specific activity of ribosomal RNA prevented a thorough analysis of its synthesis. Cellular impermeability at T9 (pH 8.6) resulted in minimal uptake of label, and an analysis of pulse-labeled transcripts was impossible. A comparison of the percantage of polysomal material indicate, however, that these cells were at least as active in translation as cells pulse-labeled at pH 6.0.
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