FAL1P IS AN ESSENTIAL DEAD-BOX PROTEIN INVOLVED IN 40S-RIBOSOMAL-SUBUNIT BIOGENESIS IN SACCHAROMYCES-CEREVISIAE

A previously uncharacterized Saccharomyces cerevisiae gene, FAL1, was found by sequence comparison as a homolog of the eukaryotic translatio n initiation factor 4A (eIF4A). Fal1p has 55% identity and 73% similar ity on the amino acid level to yeast eIF4A, the prototype of ATP-depen dent RNA helicases of the DEAD-box protein family. Although clearly gr ouped in the eIF4A subfamily, the essential Fal1p displays a different subcellular function and localization. An HA epitope-tagged Fal1p is localized predominantly in the nucleolus, Polysome analyses in a tempe rature-sensitive fal1-1 mutant and a Fal1p-depleted strain reveal a de crease in the number of 40S ribosomal subunits, Furthermore, these str ains are hypersensitive to the aminoglycoside antibiotics paromomycin and neomycin. Pulse-chase labeling of pre-rRNA and steady-state-level analysis of pre-rRNAs and mature rRNAs by Northern hybridization and p rimer extension in the Fal1p-depleted strain show that Fal1p is requir ed for pre-rRNA processing at sites A(0), A(1), and A(2). Consequently , depletion of Fal1p leads to decreased 18S rRNA levels and to an over all deficit in 40S ribosomal subunits, Together, these results implica te Fal1p in the 18S rRNA maturation pathway rather than in translation initiation.