# Knockout of the mitoribosome rescue factors Ict1 or Mtrfr is viable in zebrafish but not mice: compensatory mechanisms underlying each factor's loss

## TL;DR

Zebrafish can survive without Ict1 or Mtrfr, but mice cannot, suggesting compensatory mechanisms in zebrafish.

## Contribution

The study reveals compensatory mechanisms in zebrafish allowing survival despite loss of mitoribosome rescue factors.

## Key findings

- Zebrafish knockout lines for ict1 and mtrfr are viable and fertile.
- Zebrafish Ict1 lacks a key motif found in other vertebrates, suggesting a different functional role.
- Zebrafish show altered mitochondrial morphology and stress survival in knockout lines.

## Abstract

The mitochondrial translation system contains two ribosome rescue factors, ICT1 and MTRFR (C12orf65), which hydrolyze peptidyl‐tRNA in stalled ribosomes. ICT1 also functions as a ribosomal protein of the mitochondrial large ribosomal subunit (mtLSU) in mice and humans, and its deletion is lethal. In contrast, MTRFR does not share this role. Although loss‐of‐function mutations in MTRFR have been linked to human mitochondrial diseases, data on this association in other vertebrates are lacking. Here, attempts to generate Mtrfr knockout mice were unsuccessful. However, knockout zebrafish lines were successfully generated for both ict1 and mtrfr (ict1
−/− and mtrfr
−/−). Both knockout lines appeared healthy and fertile. ict1
−/−, mtrfr
−/−, and wild‐type adult caudal fin cells showed significant differences in mitochondrial morphology. The ict1 deletion affected the network properties more than the number of individuals and networks, whereas the mtrfr deletion exhibited the opposite effect. Additionally, the survival rates of the knockout line larvae were significantly lower than those of the wild‐type larvae under starvation conditions. These results suggest that ict1 and mtrfr are required for survival under specific stress conditions, whereas ict1
−/− and mtrfr
−/− involve different compensatory mechanisms in response to loss of either factor under nonstress conditions. Ict1 proteins from all teleosts, including zebrafish, lack the N‐terminal mtLSU‐binding motif found in most metazoans, suggesting that Ict1 does not function as a ribosomal protein in teleosts. Thus, Mtrfr may partially compensate for the loss of Ict1. In conclusion, zebrafish appear to exemplify a limited category of vertebrates capable of enduring genetic abnormalities in ict1 or mtrfr.

Mitochondria contain two mitoribosome rescue factors, ICT1 and MTRFR (C12orf65). ICT1 also functions as a mitoribosomal protein in mice and humans, and its loss is lethal. Although Mtrfr knockout mice could not be generated, knockout zebrafish lines for ict1 and mtrfr were established. Zebrafish Ict1 lack the N‐terminal mtLSU‐binding (YSLDK) motif; zebrafish represent a limited category of vertebrates capable of tolerating defects in ict1 or mtrfr.