Plant MSH1 Protein Functions as Mismatch-Directed Nuclease for Organelle Genome Maintenance
Researchers have characterized how the Arabidopsis thaliana protein AtMsh1 recognizes and cleaves mismatched or damaged DNA in plant organelles, acting as a minimal mismatch repair system. Plants are known to maintain unusually low mutation rates in their organellar genomes compared to other eukaryotes, and MSH1 disruption reverses this advantage, but the precise mechanism had been unknown. This study clarifies that AtMsh1 couples mismatch recognition with targeted DNA cleavage, potentially explaining how plants preserve organellar genome integrity across generations.
A new preprint study on bioRxiv reports that the plant protein AtMsh1, encoded by the MSH1 gene in Arabidopsis thaliana, functions as a mismatch-directed nuclease that both recognizes and cleaves damaged or mismatched double-stranded DNA in plant organelles. The enzyme is structurally modular, combining an N-terminal MutS DNA mismatch repair domain with a C-terminal GYIYIG nuclease domain. AtMsh1 efficiently processes DNA lesions caused by oxidative damage or deamination, showing a particularly strong preference for uracil-guanine (U:G) mismatches. Cleavage requires ATP and divalent metal cofactors such as magnesium, and occurs at defined positions — approximately nine nucleotides 5' of the mismatch on the affected strand and twelve nucleotides 3' on the complementary strand — producing staggered ends with three-nucleotide overhangs. The authors propose that these double-strand breaks are subsequently processed by exonucleases, generating single-stranded 3' overhangs suitable for homologous recombination repair and gene conversion, thereby removing the lesion. In the presence of manganese instead of magnesium, the enzyme loses its positional specificity and cleaves DNA nonspecifically, a notable mechanistic distinction. These findings establish AtMsh1 as a self-contained, minimal mismatch repair system and offer a mechanistic explanation for the exceptionally low organellar mutation rates observed in plants.
What's missing
As a preprint, this work has not yet undergone formal peer review, and independent replication of the biochemical findings has not been reported. The study characterizes AtMsh1 activity in vitro, and it remains to be demonstrated whether the proposed repair pathway — including exonuclease processing and homologous recombination — operates with the same specificity in vivo in plant organelles. It is also unclear whether the manganese-dependent nonspecific cleavage activity has any physiological relevance or represents an experimental artifact.
What different sources said
- bioRxivCenter
Plant MutS Homolog 1 is a mismatch-directed nuclease required for organelle genome maintenance
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