Assembly of the Archaeal Box C/D sRNP Can Occur via Alternative Pathways and Requires Temperature-facilitated sRNA Remodeling

Assembly of the Archaeal Box C/D sRNP Can Occur via Alternative Pathways and Requires Temperature-facilitated sRNA Remodeling
Received 12 May 2006; revised 21 July 2006; accepted 29 July 2006. Edited by J. Doudna. Available online 4 August 2006.
Keith T. Gagnona, Xinxin Zhanga, Paul F. Agrisa and E. Stuart Maxwell, a
Journal of Molecular Biology
Volume 362, Issue 5 , 6 October 2006
Copyright ? 2006 Published by Elsevier Ltd.
aDepartment of Molecular and Structural Biochemistry, North Carolina State University, Raleigh, NC 27695-7622, USA
Archaeal dual-guide box C/D small nucleolar RNA-like RNAs (sRNAs) bind three core proteins in sequential order at both terminal box C/D and internal C'/D' motifs to assemble two ribonuclear protein (RNP) complexes active in guiding nucleotide methylation. Experiments have investigated the process of box C/D sRNP assembly and the resultant changes in sRNA structure or ?remodeling? as a consequence of sRNP core protein binding. Hierarchical assembly of the Methanocaldococcus jannaschii sR8 box C/D sRNP is a temperature-dependent process with binding of L7 and Nop56/58 core proteins to the sRNA requiring elevated temperature to facilitate necessary RNA structural dynamics. Circular dichroism (CD) spectroscopy and RNA thermal denaturation revealed an increased order and stability of sRNA folded structure as a result of L7 binding. Subsequent binding of the Nop56/58 and fibrillarin core proteins to the L7?sRNA complex further remodeled sRNA structure. Assessment of sR8 guide region accessibility using complementary RNA oligonucleotide probes revealed significant changes in guide region structure during sRNP assembly. A second dual-guide box C/D sRNA from M. jannaschii, sR6, also exhibited RNA remodeling during temperature-dependent sRNP assembly, although core protein binding was affected by sR6's distinct folded structure. Interestingly, the sR6 sRNP followed an alternative assembly pathway, with both guide regions being continuously exposed during sRNP assembly. Further experiments using sR8 mutants possessing alternative guide regions demonstrated that sRNA folded structure induced by specific guide sequences impacted the sRNP assembly pathway. Nevertheless, assembled sRNPs were active for sRNA-guided methylation independent of the pathway followed. Thus, RNA remodeling appears to be a common and requisite feature of archaeal dual-guide box C/D sRNP assembly and formation of the mature sRNP can follow different assembly pathways in generating catalytically active complexes.
Keywords: archaeal sRNA; box C/D RNA; RNA-guided nucleotide methylation; RNA remodeling; RNP assembly
Abbreviations: snoRNA, small nucleolar RNA; sRNA, box C/D snoRNA-like RNAs; sRNP, small ribonuclear protein; K-turn, kink-turn; EMSA, electrophoretic mobility-shift analysis
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