Substrate selectivity by the exonuclease Rrp6p.

Area of Science:

• Molecular biology
• RNA metabolism
• Enzymology

Background:

• The exoribonuclease Rrp6p plays a crucial role in RNA degradation within the cell nucleus.
• Rrp6p processes a wide array of RNA substrates but exhibits selectivity, avoiding indiscriminate degradation of all RNAs.
• The precise mechanisms underlying Rrp6p's substrate selectivity remain largely unelucidated.

Purpose of the Study:

• To investigate the in vitro binding and degradation kinetics of Rrp6p with RNA substrates at single-nucleotide resolution.
• To identify the molecular determinants of Rrp6p's substrate selectivity.
• To understand how Rrp6p discriminates against specific RNA sequences, particularly those resembling 3' termini of uncharged tRNAs.

Main Methods:

• Quantitative in vitro assays measuring Rrp6p-RNA binding affinities.
• Kinetic analysis of Rrp6p-mediated RNA degradation.
• Single-nucleotide resolution mapping of Rrp6p activity on diverse RNA substrates.

Main Results:

• Rrp6p demonstrates intrinsic substrate selectivity, enabling discrimination against specific RNAs.
• RNA length and the terminal four 3'-nucleotides are key determinants of Rrp6p substrate selectivity.
• Pronounced discrimination was observed against RNAs with a CCA-3' terminal sequence, similar to uncharged tRNA 3' ends.

Conclusions:

• Rrp6p utilizes substrate selectivity primarily to avoid degrading specific RNAs, rather than to preferentially bind certain targets.
• This selective avoidance mechanism allows Rrp6p to process diverse RNA substrates while protecting essential RNA subsets, such as uncharged tRNAs.
• The findings provide novel insights into the regulatory mechanisms governing nuclear RNA decay pathways.