Novel stress-responsive genes EMG1 and NOP14 encode conserved, interactingproteins required for 40S ribosome biogenesis

Under stressful conditions organisms adjust the synthesis, processing, and trafficking of molecules to allow survival from and recovery after stress. In baker's yeast Saccharomyces cerevisiae, the cellular production of ribos omes is tightly matched with environmental conditions and nutrient availabi lity through coordinate transcriptional regulation of genes involved in rib osome biogenesis. On the basis of stress-responsive gene expression and fun ctional studies, we have identified a novel, evolutionarily conserved gene, EMG1, that has similar stress-responsive gene expression patterns as ribos omal protein genes and is required for the biogenesis of the 40S ribosomal subunit. The Emg1 protein is distributed throughout the cell; however, its nuclear localization depends on physical interaction with a newly character ized nucleolar protein, Nop14. Yeast depleted of Nop14 or harboring a tempe rature-sensitive allele of emg1 have selectively reduced levels of the 20S pre-rRNA and mature18S rRNA and diminished cellular levels of the 40S ribos omal subunit. Neither Emg1 nor Nop14 contain any characterized functional m otifs; however, isolation and functional analyses of mammalian orthologues of Emg1 and Nop14 suggest that these proteins are functionally conserved am ong eukaryotes. We conclude that Emg1 and Nop14 are novel proteins whose in teraction is required for the maturation of the 18S rRNA and for 40S riboso me production.