FIG. 3.

The translation of rp mRNAs is repressed upon growth arrest of eIF-4E-overexpressing cells. Cytoplasmic extracts from P2 cells that were either untreated (Con) or growth arrested (+) by 24-h treatment with either aphidicolin or hydroxyurea, or 24 h after withdrawal of the drug (W), were centrifuged through sucrose gradients and separated into polysomal (P) and subpolysomal (S) fractions. Poly(A)⁺ mRNA from equivalent aliquots of these fractions was analyzed by Northern blot hybridization with the probes indicated at the left. Because of variations in the amount of cytoplasmic extracts separated in each gradient and different exposure times for the + or W lanes, even for the same drug or the same probe, quantitative comparisons of the autoradiographic signal can be made only between the polysomal and subpolysomal fractions of the same gradient. The percentage of mRNA in polysomal fractions was assessed as described in Material and Methods, and the results presented at the right are an average of the number of experiments indicated in parentheses. The SEM in all experiments with three or more measurments was less than 10%. The lower band in actin-probed lane S of hydroxyurea-withdrawal cells is a contamination and not a fast running actin mRNA.