CLN3 expression is sufficient to restore G1-to-S-phase progression in Saccharomyces cerevisiae mutants defective in translation initiation factor eIF4E

The essential cap-binding protein (eIF4E) of Saccharomycescerevisiae is encoded by the CDC33 (wild-type) gene, originally isolated as a mutant, cdc33-1, which arrests growth in the G₁ phase of the cell cycle at 37 °C. We show that other cdc33 mutants also arrest in G₁. One of the first events required for G₁-to-S-phase progression is the increased expression of cyclin 3. Constructs carrying the 5ʹ-untranslated region of CLN3 fused to lacZ exhibit weak reporter activity, which is significantly decreased in a cdc33-1 mutant, implying that CLN3 mRNA is an inefficiently translated mRNA that is sensitive to perturbations in the translation machinery. A cdc33-1 strain expressing either stable Cln3p (Cln3-1p) or a hybrid UBI4 5ʹ-CLN3 mRNA, whose translation displays decreased dependence on eIF4E, arrested randomly in the cell cycle. In these cells CLN2 mRNA levels remained high, indicating that Cln3p activity is maintained. Induction of a hybrid UBI4 5ʹ-CLN3 message in a cdc33-1 mutant previously arrested in G₁ also caused entry into a new cell cycle. We conclude that eIF4E activity in the G₁-phase is critical in allowing sufficient Cln3p activity to enable yeast cells to enter a new cell cycle.