CLN3 expression is sufficient to restore G(1)-to-S-phase progression in Saccharomyces cerevisiae mutants defective in translation initiation factor eIF4E

The essential cap-binding protein (eIF4E) of Saccharomyces cerevisiae is en coded by the CDC33 (wild-type) gene, originally isolated as a mutant, cdc33 -1, which arrests growth in the G(1)-phase of the cell cycle at 37 degrees C. We show that other cdc33 mutants also arrest in G(1). One of the first e vents required for G(1)-to-S-phase progression is the increased expression of cyclin 3. Constructs carrying the 5'-untranslated region of CLN3 fused t o lacZ exhibit weak reporter activity, which is significantly decreased in a cdc33-1 mutant, implying that CLN3 mRNA is an inefficiently translated mR NA that is sensitive to perturbations in the translation machinery. A cdc33 -1 strain expressing either stable Cln3p (Cln3-1p) or a hybrid UBI4 5'-CLN3 mRNA, whose translation displays decreased dependence on eIF4E, arrested r andomly in the cell cycle. In these cells CLN2 mRNA levels remained high, i ndicating that Cln3p activity is maintained. Induction of a hybrid UBI4 5'- CLN3 message in a cdc33-1 mutant previously arrested in G(1) also caused en try into a new cell cycle. We conclude that eIF4E activity in the G(1)-phas e is critical in allowing sufficient Cln3p activity to enable yeast cells t o enter a new cell cycle.