Box C/D sRNA stem ends act as stabilizing anchors for box C/D di-sRNPs

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Ribosomal RNA (rRNA) modifications are vital for ribosome function across all life.
• Box C/D small (nucleolar) ribonucleoproteins [s(no)RNPs] are responsible for 2'-O-methylation, a key rRNA modification in Eukarya and Archaea.
• Previous structural studies (EM, NMR) established the dimeric sRNP (di-sRNP) architecture of archaeal box C/D sRNPs but could not resolve sRNA orientation.

Purpose of the Study:

• To determine the orientation of small RNAs (sRNAs) within the box C/D di-sRNP complex from *Methanococcus jannaschii*.
• To investigate the structural basis for the stability of the di-sRNP architecture.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to reconstruct the 3D structure of the *M. jannaschii* box C/D di-sRNP.
• Biochemical and structural analyses of sRNPs reconstituted with mutant sRNAs.

Main Results:

• Cryo-EM data revealed a parallel orientation of the two sRNAs within the di-sRNP.
• Analysis of mutant sRNAs suggested a potential interaction between the stem ends of the parallel sRNAs.
• This parallel arrangement likely juxtaposes the sRNA stem ends, facilitating a stabilizing interaction crucial for di-sRNP integrity.

Conclusions:

• The study elucidates the sRNA orientation in archaeal box C/D di-sRNPs, revealing a parallel arrangement.
• This parallel orientation is proposed to stabilize the di-sRNP structure through interactions at the sRNA stem ends.
• The findings provide critical insights into the structural organization and stability mechanisms of these essential ribonucleoprotein complexes.