# Characteristics of C4-Type Zinc Finger Proteins in Tardigrades and Their Responses in Extreme Environments

## TL;DR

This study explores how C4-type zinc finger proteins in tardigrades help them survive extreme environments.

## Contribution

The paper systematically identifies and analyzes C4-type zinc finger proteins in tardigrades and links them to stress adaptation.

## Key findings

- C4-type ZFPs are widely distributed in tardigrades and grouped into six phylogenetic clusters.
- Group 5 ZFPs show stress-related promoter elements and are downregulated after irradiation.
- Downregulation of Group 5 ZFPs may help prioritize damage repair over growth during stress.

## Abstract

The regulatory role of zinc finger proteins is crucial for the development of extreme environmental tolerance in numerous species. Tardigrades, as multicellular animals capable of withstanding multiple extreme conditions, have yet to undergo systematic identification and analysis of their zinc finger proteins. This study first analysed the distribution characteristics of zinc finger proteins across four species of tardigrades. We systematically characterised the family features of C4-type ZFPs in these four species and explored their regulatory roles in extreme environmental adaptation. Statistical genomic investigation reveals a wide distribution of the C4-type ZFP family among tardigrades. Phylogenetically, they separate into six distinct groups, with Group 5 emerging as a functionally specialised branch enriched in stress-responsive promoter elements. Molecular evolutionary evidence points to tandem duplication as key to this branch’s expansion and functional innovation, while also highlighting the pervasive purifying selection across this family. Transcriptomic analysis revealed that a specific subset of C4-type ZFP genes belonging to phylogenetic Group 5 in both H. henanensis and H. exemplaris were downregulated after irradiation. Functional enrichment indicates that these genes are linked to nuclear receptor transcription factor activity and the negative regulation of NF-κB signalling. We propose that their coordinated downregulation may represent a conserved stress-adaptive response, potentially derepressing NF-κB and reprioritizing resources toward damage repair over growth. This study provides a foundation and key clues for research aimed at elucidating the functional mechanisms of C4-type ZFPs in tardigrade extremotolerance.