Dhr1p, a putative DEAH-Box RNA helicase, is associated with the box C+D snoRNP U3

Putative RNA helicases are involved in most aspects of gene expression. All previously characterized members of the DEAH-box family of putative RNA he licases are involved in pre-mRNA splicing. Here we report the analysis of t wo novel DEAH-box RNA helicases, Dhr1p and Dhr2p, that were found to be pre dominantly nucleolar. Both genes are essential for viability, and MET-regul ated alleles were therefore created. Depletion of Dhr1p or Dhr2p had no det ectable effect on pre-mRNA splicing in vivo or in vitro. Both Dhr1p and Dhr 2p were, however, required for 18S rRNA synthesis. Depletion of Dhr2p inhib ited pre-rRNA cleavage at sites A(0), A(1), and A(2), while Dhr1p depletion inhibited cleavage at sites A(1) and A(2). No coprecipitation of snoRNAs w as detected with ProtA-Dhr2p, but Dhr1p-ProtA was stably associated with th e U3 snoRNA. Depletion of Dhr1p inhibited processing steps that require bas e pairing of U3 to the 5' end of the 18S rRNA. We speculate that Dhr1p is t argeted to the preribosomal particles by the U3-18S rRNA interaction and is required for the structural reorganization of the rRNA during formation of the central pseudoknot.