The Cotranscriptional Assembly of snoRNPs Controls the Biosynthesis of H/ACA snoRNAs in Saccharomyces cerevisiae

Abstract

The carboxy-terminal domain (CTD) of RNA polymerase II large subunit acts as a platform to assemble the RNA processing machinery in a controlled way throughout the transcription cycle. In yeast, recent findings revealed a physical connection between phospho-CTD, generated by the Ctk1p kinase, and protein factors having a function in small nucleolar RNA (snoRNA) biogenesis. The snoRNAs represent a large family of polymerase II noncoding transcripts that are associated with highly conserved polypeptides to form stable ribonucleoprotein particles (snoRNPs). In this work, we have studied the biogenesis of the snoRNPs belonging to the box H/ACA class. We report that the assembly factor Naf1p and the core components Cbf5p and Nhp2p are recruited on H/ACA snoRNA genes very early during transcription. We also show that the cotranscriptional recruitment of Naf1p and Cbf5p is Ctk1p dependent and that Ctk1p and Cbf5p are required for preventing the readthrough into the snoRNA downstream genes. All these data suggest that proper cotranscriptional snoRNP assembly controls 3′-end formation of snoRNAs and, consequently, the release of a functional particle.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/.

ACKNOWLEDGMENTS

We are grateful to Stephen Buratowski for generously providing the YSB854 yeast strain and for helpful suggestions on ChIP analysis. We thank M. Arceci and G. Ricci for skillful technical help.

This work was partially supported by grants from MURST (FIRB-p.n. RBNE015MPB and RBNE01KXC9-, PRIN-Cofin, and Centro di Eccellenza BEMM).