The Features and Regulation of Co-transcriptional Splicing in Arabidopsis

Precursor mRNA(pre-mRNA)splicing is essential for gene expression in most eukaryotic organisms.Previous studies from mammals,Drosophila,and yeast show that the majority of splicing events occurs co-transcriptionally.In plants,however,the features of co-transcriptional splicing(CTS)and its regulation still remain largely unknown.Here,we used chromatin-bound RNA sequencing to study CTS in Arabidopsis thaliana.We found that CTS is widespread in Arabidopsis seedlings,with a large proportion of alternative splicing events determined co-transcriptionally.CTS efficiency correlated with gene expression level,the chromatin landscape and,most surprisingly,the number of introns and exons of individual genes,but is independent of gene length.In combination with enhanced crosslinking and immunoprecipitation sequencing analysis,we further showed that the hnRNP-like proteins RZ-1B and RZ-1C promote efficient CTS globally through direct binding,frequently to exonic sequences.Notably,this general effect of RZ-1B/1C on splicing promotion is mainly observed at the chromatin level,not at the mRNA level.RZ-1C promotes CTS of multiple-exon genes in association with its binding to regions both proximal and distal to the regulated introns.We propose that RZ-1C promotes efficient CTS of genes with multiple exons through cooperative interactions with many exons,introns,and splicing factors.Our work thus reveals important features of CTS in plants and provides methodologies for the investigation of CTS and RNA-binding proteins in plants.

Global Co-transcriptional Splicing in Arabidopsis and the Correlation with Splicing Regulation in Mature RNAs

RNA splicing and spliceosome assembly in eukaryotes occur mainly during transcription.However,co-transcriptional splicing has not yet been explored in plants.Here,we built transcriptomes of nascent chromatin RNAs in Arabidopsis thaliana and showed that nearly all introns undergo co-transcriptional splicing,which occurs with higher efficiency for introns in protein-coding genes than for those in noncoding RNAs.Total intron number and intron position are two predominant features that correlate with co-transcriptional splicing efficiency,and introns with alternative 5′or 3′splice sites are less efficiently spliced.Furthermore,we found that mutations in genes encoding trans-acting proteins lead to more introns with increased splicing defects in nascent RNAs than in mature RNAs,and that introns with increased splicing defects in mature RNAs are inefficiently spliced at the co-transcriptional level.Collectively,our results not only uncovered widespread co-transcriptional splicing in Arabidopsis but also identified features that may affect or be affected by co-transcriptional splicing efficiency.

Lack of transcriptional coordination between mitochondrial and nuclear oxidative phosphorylation genes in the presence of two divergent mitochondrial genomes

In most eukaryotes,oxidative phosphorylation(OXPHOS)is the main energy production process and it involves both mitochondrial and nuclear genomes.The close interaction between the two genomes is critical for the coordinated function of the OXPHOS process.Some bivalves show doubly uniparental inheritance(DUI)of mitochondria,where two highly divergent mitochondrial genomes,one inherited through eggs(F-type)and the other through sperm(M-type),coexist in the same individual.However,it remains a puzzle how nuclear OXPHOS genes coordinate with two divergent mitochondrial genomes in DUI species.In this study,we compared transcription,polymorphism,and synonymous codon usage in the mitochondrial and nuclear OXPHOS genes of the DUI species Ruditapes philippinarum using sex-and tissue-specific transcriptomes.Mitochondrial and nuclear OXPHOS genes showed different transcription profiles.Strong co-transcription signal was observed within mitochondrial(separate for F-and M-type)and within nuclear OXPHOS genes but the signal was weak or absent between mitochondrial and nuclear OXPHOS genes,suggesting that the coordination between mitochondrial and nuclear OXPHOS subunits is not achieved transcriptionally.McDonald-Kreitman and frequency-spectrum based tests indicated that M-type OXPHOS genes deviated significantly from neutrality,and that F-type and M-type OXPHOS genes undergo different selection patterns.Codon usage analysis revealed that mutation bias and translational selection were the major factors affecting the codon usage bias in different OXPHOS genes,nevertheless,translational selection in mitochondrial OXPHOS genes appears to be less efficient than nuclear OXPHOS genes.Therefore,we speculate that the coordination between OXPHOS genes may involve post-transcriptional/translational regulation.

Folding Behaviors of Purine Riboswitch Aptamers

Most riboswitches are characterized by two components, an aptamer domain that folds into a unique ligand binding pocket to interact with the ligand, and an expression platform that converts folding changes in the aptamer into changes in gene expression. Using the recently developed systematic helix-based computational method, we theoretically studied the refolding and co-transcriptional folding behaviors of the purine riboswitch aptamers from Bacillus subtilis xpt-pbu X guanine riboswitch and Vibrio vulnificus add adenine riboswitch. Despite several intermediate structures persisting a short time during the transcription, helices P2, P3 and P1 fold in turn for both aptamers. Although some misfolded structures are observed during the refolding process, the RNAs can fold into the ligand binding pocket structure containing helices P2, P3 and P1 within a few seconds, suggesting the aptamer domains are highly evolved. The purine riboswitch aptamers can quickly fold into the ligand binding pocket structure even at a high transcription speed, possibly because formation of this structure is the necessary prerequisite for the riboswitch to bind its ligand and then regulate relevant gene expression.