# Sni445 recruits box C/D snoRNPs snR4 and snR45 to guide ribosomal RNA acetylation by Kre33

## TL;DR

The protein Sni445 helps guide specific RNA modifications in yeast ribosomes by recruiting snoRNPs and enabling acetylation of rRNA.

## Contribution

Sni445 is identified as a novel auxiliary component of snR4 and snR45 snoRNPs, essential for their stable incorporation and acetylation activity.

## Key findings

- Sni445 associates with snR4 and snR45 snoRNPs and is required for their stable incorporation into 90S pre-ribosomes.
- Sni445 interacts with Kre33 and is essential for acetylation of rRNA residues C1280 and C1773.
- Genetic interactions link Sni445 and snoRNAs to ribosomal proteins near acetylation sites.

## Abstract

Eukaryotic ribosome synthesis is a highly complex, multistep process that is best characterized in the yeast Saccharomyces cerevisiae. It is orchestrated by >200 ribosome assembly factors and 75 small nucleolar ribonucleoproteins (snoRNPs), which guide site-specific chemical modifications of precursor rRNA (pre-rRNA). While canonical box C/D snoRNPs guide 2′-O-methylation, the atypical box C/D snoRNPs snR4 and snR45 guide acetylation of 18S rRNA residues C1280 and C1773, respectively, catalyzed by the acetyltransferase Kre33. Here, we identify and characterize Ynl050c/Sni445 as a novel ribosome assembly factor and previously unrecognized auxiliary component of the snR4 and snR45 box C/D snoRNPs. Sni445 associates with snR4 and snR45 in their free form and is required for their stable incorporation into 90S pre-ribosomes. Genetic interactions link Sni445 and the snR4 and snR45 snoRNAs to ribosomal proteins Rps20 (uS10) and Rps14 (uS11), which are positioned near the respective acetylation sites in the 40S subunit. Moreover, Sni445 physically interacts with Kre33 within the 90S pre-ribosome, and its absence abolishes acetylation of C1280 and C1773. Our findings suggest that Sni445 facilitates the recruitment of snR4 and snR45 snoRNPs to 90S particles and might promote their interaction with Kre33, thereby enabling the site-specific acetylation of 18S rRNA by Kre33.

Graphical Abstract