YRA1, AN ESSENTIAL SACCHAROMYCES-CEREVISIAE GENE, ENCODES A NOVEL NUCLEAR-PROTEIN WITH RNA ANNEALING ACTIVITY

The complexity of eukaryotic mRNA processing suggests a need for certa in factors, called RNA chaperones, that can modulate RNA secondary str ucture as well as the interactions between pre-mRNA and trans-acting c omponents. To identify factors that may fulfill this role in the yeast Saccharomyces cerevisiae, we fractionated whole-cell extracts and ass ayed for activity that could facilitate a specific RNA-RNA annealing r eaction. We detected one strong RNA annealing activity and purified it to homogeneity. This previously undescribed factor, Yra1p, is localiz ed to the nucleus; its sequence contains one RNP-motif RNA-binding dom ain. The YRA1 gene contains a 766-nt intron, the second-largest identi fied in this organism, and Yra1p serves an essential, nonredundant fun ction. Taken together, our findings indicate that Yra1p is likely to h ave an important role in S. cerevisiae nuclear pre-mRNA metabolism.