A fission yeast general translation factor reveals links between protein synthesis and cell cycle controls

In two independent screens we isolated fission yeast mutations with phenoty pes suggesting defects ill B-cyclin function or expression. These mutations define a single gene which we call ded1, We show that ded1 encodes a gener al translation factor that is related in sequence and function to RNA helic ases required for translation in other species. Levels of the B-cyclins Cig 2 and Cdc13 are dramatically reduced upon inactivation of Ded1, and this re duction is independent of degradation by the anaphase promoting complex. Wh en a ded1 mutant is grown under semi-restrictive conditions, the translatio n of Cig2 land to a lesser extent Cdc13), is impaired relative to other pro teins. We show that B-cyclin translation is specifically inhibited upon nit rogen starvation of wild-type cells, when B-cyclin/Cdc2 inactivation is a p rerequisite for G(1) arrest and subsequent mating, Our data suggest that tr anslational inhibition of B-cyclin expression represents a third mechanism, in addition to cyclin degradation and Rum1 inhibition, that contributes to Cdc2 inactivation as cells exit from the mitotic cell cycle and prepare fo r meiosis.