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The DEAH-box Helicase Dhr1 Dissociates U3 from the Pre-rRNA to Promote Formation of the Central Pseudoknot

2015; Public Library of Science; Volume: 13; Issue: 2 Linguagem: Inglês

10.1371/journal.pbio.1002083

1545-7885

Richa Sardana, Xin Liu, Sander Granneman, Jieyi Zhu, Michael Gill, Ophelia Papoulas, Edward M. Marcotte, David Tollervey, Carl C. Correll, Alan M. Johnson,

RNA Research and Splicing

In eukaryotes, the highly conserved U3 small nucleolar RNA (snoRNA) base-pairs to multiple sites in the pre-ribosomal RNA (pre-rRNA) to promote early cleavage and folding events. Binding of the U3 box A region to the pre-rRNA is mutually exclusive with folding of the central pseudoknot (CPK), a universally conserved rRNA structure of the small ribosomal subunit essential for protein synthesis. Here, we report that the DEAH-box helicase Dhr1 (Ecm16) is responsible for displacing U3. An active site mutant of Dhr1 blocked release of U3 from the pre-ribosome, thereby trapping a pre-40S particle. This particle had not yet achieved its mature structure because it contained U3, pre-rRNA, and a number of early-acting ribosome synthesis factors but noticeably lacked ribosomal proteins (r-proteins) that surround the CPK. Dhr1 was cross-linked in vivo to the pre-rRNA and to U3 sequences flanking regions that base-pair to the pre-rRNA including those that form the CPK. Point mutations in the box A region of U3 suppressed a cold-sensitive mutation of Dhr1, strongly indicating that U3 is an in vivo substrate of Dhr1. To support the conclusions derived from in vivo analysis we showed that Dhr1 unwinds U3-18S duplexes in vitro by using a mechanism reminiscent of DEAD box proteins.