A YEAST GLUTAMINE TRANSFER-RNA SIGNALS NITROGEN STATUS FOR REGULATIONOF DIMORPHIC GROWTH AND SPORULATION

Dimorphic growth of the budding yeast Saccharomyces cerevisiae is regu lated by the quality of the nitrogen supply. On a preferred nitrogen s ource diploid cells grow as ellipsoidal cells by using a bipolar patte rn of budding, whereas on a poor nitrogen source a unipolar pattern of budding is adopted, resulting in extended pseudohyphal chains of fila mentous cells. Here we report that the quality of the nitrogen source is signaled by the glutamine tRNA isoform with a 5'-CUG anticodon (tRN A(CUG)). Mutations that alter this tRNA impair assessment of the nitro gen supply without measurably affecting protein synthesis, so that mut ant cells display pseudohyphal growth even on a preferred nitrogen sou rce, The nitrogen status for other nitrogen-responsive processes such as catabolic gene expression and sporulation also is signaled by this tRNA: mutant cells inappropriately induce the nitrogen-repressed gene CAR1 and undergo precocious sporulation in nitrogen-rich media. Theref ore, in addition to its role in mRNA translation, this tRNA also trans duces nitrogen signals that regulate development.