DBP3P, A PUTATIVE RNA HELICASE IN SACCHAROMYCES-CEREVISIAE, IS REQUIRED FOR EFFICIENT PRE-RIBOSOMAL-RNA PROCESSING PREDOMINANTLY AT SITE A(3)

In Saccharomyces cerevisiae, ribosomal biogenesis takes place primaril y in the nucleolus, in which a single 35S precursor rRNA (pre-rRNA) is first transcribed and sequentially processed into 25S, 5.8S, and 18S mature rRNAs, leading to the formation of the 40S and 60S ribosomal su bunits. Although many components involved in this process have been id entified, our understanding of this important cellular process remains limited. Here we report that one of the evolutionarily conserved DEAD -box protein genes in yeast, DBP3, is required for optimal ribosomal b iogenesis. DBP3 encodes a putative RNA helicase, Dbp3p, of 523 amino a cids in length, which bears a highly charged amino terminus consisting of 10 tandem lysine-lysine-X repeats ([KKX] repeats). Disruption of D BP3 is not lethal but yields a slow-growth phenotype. This genetic dep letion of Dbp3p results in a deficiency of 60S ribosomal subunits and a delayed synthesis of the mature 25S rRNA, which is caused by a promi nent kinetic delay in pre-rRNA processing at site A(3) and to a lesser extent at sites A(2) and A(0). These data suggest that Dbp3p may dire ctly or indirectly facilitate RNase MRP cleavage at site A,. The direc t involvement of Dbp3p in ribosomal biogenesis is supported by the fin ding that Dbp3p is localized predominantly in the nucleolus. In additi on, we show that the [KKX] repeats are dispensable for Dbp3p's functio n in ribosomal biogenesis but are required for its proper localization . The [KKX] repeats thus represent a novel signaling motif for nuclear localization and/or retention.