🧬 PubMed RNA Editing Daily Digest

CRISPR/Cas9-based genome editing in the model bryophyte Physcomitrium patens (commonly known as Physcomitrella) is widely used for gene knockout via small insertions or deletions (indels). In this study, we developed an efficient dual-gRNA system capable of producing large, targeted deletions across multiple genes, enabling straightforward detection by gel electrophoresis and simultaneous multi-gene knockout. We first compared the efficiency of polycistronic tRNA-gRNA arrays to conventional gRNA constructs expressed under individual promoters, using the checkpoint protein gene MAD2 as a target. We found that a polycistronic construct doubled the frequency of large gene deletions compared to a conventional design. We then demonstrated that simultaneous deletion of two or four genes, targeting the katanin and TPX2 gene families, respectively, can be achieved in a single transformation event. The polycistronic system also increased deletion frequencies in the multiplex context, with up to 42% efficiency for individual genes and successful recovery of quadruple mutants. As a drawback, we confirmed that deletion efficiency varied substantially among individual gRNA pairs, indicating that gRNA design remains a critical factor in multiplex editing. This study establishes a fast and efficient framework for simultaneous removal of multiple genes in Physcomitrella, providing a practical alternative to homologous recombination-based methods for functional and applied studies.

Genomic RNA (gRNA) dimerization is essential for retroviral replication. In the gRNA of human immunodeficiency virus (HIV-1), the hairpin-like dimerization initiation sequence (DIS) forms a kissing-complex (KC) with the DIS sequence in another gRNA, which later converts into a stable extended-duplex (ED). Using coarse-grained simulations, we mapped the transition of HIV-1 DIS RNA hairpins (HPs) to ED and identified multiple intermediates beyond the KC. The KC has an anionic pocket stabilized through the condensation of Mg

Garlic (Allium sativum) is an important crop with significant value in both agriculture and medicine, yet its mitochondrial genome remains uncharacterized. This gap has limited understanding of organellar evolution and genomic diversity. The mitogenome assembled with Illumina and PacBio revealed a complex, multipartite architecture spanning 548,160 bp with six contigs, a size that is within the common range for angiosperm mitochondrial genomes. The structure demonstrated considerable plasticity, and was characterized by abundant repetitive sequences. Annotation identified 25 protein-coding genes, 14 tRNAs, and three rRNAs, representing a conserved gene set. Extensive chloroplast-to-mitochondrion DNA transfer was observed, with 38 homologous fragments totaling 33.6 kb that included functionally intact genes. Codon usage analysis revealed a pronounced A/U-ending preference in synonymous codons. Additionally, 494C-to-U RNA editing sites were predicted, indicating significant concentrations in NADH dehydrogenase genes. Phylogenetic analysis based on 23 conserved mitochondrial genes robustly resolved A. sativum as sister to Allium fistulosum. This study presents the first complete mitochondrial genome of A. sativum, which reveals substantial structural complexity and dynamic evolution. This genome provides a foundational resource for further investigation into organellar genome evolution within the Allium genus.

Homing-based CRISPR-Cas9 gene drives (CCGDs) are powerful tools for genetic control of wild populations, with applications from disease eradication to species conservation. However, Cas9 alone and in a complex with guide RNA can cause double-stranded DNA breaks at off-target sites, which could increase the mutational load and lead to unintended loss-of-heterozygosity (LOH) events. These undesired effects raise potential concerns about the long-term evolutionary safety of CCGDs, but the magnitude of these effects is unknown. To measure how the presence of a CCGD or a Cas9 alone in the genome affects the rates of LOH events and de novo mutations, we carried out a mutation accumulation experiment in yeast Saccharomyces cerevisiae. We found no detectable effects on the genome-wide rates of mutations or LOH events. Our power calculations suggest that CCGD or Cas9 affect these rates by less than 30%, which is much less than natural variation for these traits in yeast. A more detailed examination shows that CCGD or Cas9 may alter the lengths and genomic distributions of LOH events, but the statistical support for these effects is weak. Thus, our results demonstrate that CCGDs impose at most a weak additional mutational burden in the yeast model. Although mutagenic effects of gene drives need to be further evaluated in other systems, our results add credence to the proposition that the evolutionary risks posed by well-designed gene drives may be acceptable.

The 2022 outbreak of monkeypox virus (MPXV), a double-stranded DNA virus, is remarkable for an unusually high number of single-nucleotide substitutions compared to earlier strains, with a strong bias toward C→T and G→A transitions consistent with the APOBEC3 cytidine deaminase activity. While APOBEC3-induced mutagenesis is well documented at the DNA level, its potential impact on MPXV RNA transcripts remains unclear. To assess whether APOBEC3 enzymes act on MPXV RNA, we analyzed RNA-seq data from infected samples. The enrichment of APOBEC signature substitutions among high-frequency mismatched positions led us to consider two possibilities: RNA editing at hotspots or fixed DNA mutations. Multiple lines of evidence support the conclusion that these substitutions arise from DNA-level mutagenesis rather than RNA editing. These include a substantial number of G→A substitutions remaining after normalization by gene strand direction, a largely neutral impact of substitutions on protein-coding sequences, the lack of positional correlation with transcriptional features or RNA secondary structure typically associated with APOBEC action hotspots, and an overlap with known genomic mutations in MPXV strains. Analysis of the nucleotide context of observed substitutions indicated that APOBEC3A or APOBEC3B was likely a driver of DNA-level mutagenesis.IMPORTANCEThe 2022 monkeypox virus (MPXV) outbreak showed an unusually high number of mutations thought to result from human antiviral enzymes of the APOBEC3 family. While such mutations have been clearly documented in the viral DNA, whether APOBEC3 also edits viral messenger RNA molecules remained unclear. In this study, we analyzed multiple publicly available MPXV RNA sequencing datasets to address this question. We found that the apparent APOBEC-like changes in RNA are best explained by fixed DNA mutations rather than active RNA editing. This finding helps clarify how MPXV evolves and adapts, suggesting that APOBEC3's role in shaping the virus likely operates at the DNA level. Understanding where and how these mutations occur provides insight into the virus's interaction with the human immune system and informs future studies on viral evolution and antiviral defenses.