Human co-transcriptional splicing kinetics and coordination revealed by direct nascent RNA sequencing

Human genes have numerous exons that are differentially spliced within pre-mRNA. Understanding how multiple splicing events are coordinated across nascent transcripts requires quantitative analyses of transient RNA processing events in living cells. We developed nanopore analysis of CO-transcriptional Processing (nano-COP), in which nascent RNAs are directly sequenced through nanopores, exposing the dynamics and patterns of RNA splicing without biases introduced by amplification. nano-COP showed that in both human and Drosophila cells, co-transcriptional splicing occurs after RNA polymerase II transcribes several kilobases of pre-mRNA, suggesting that metazoan splicing transpires distally from the transcription machinery. Inhibition of the branch-site recognition complex SF3B globally abolished co-transcriptional splicing in both species. Our findings revealed that splicing order does not strictly follow the order of transcription and is influenced by cis-regulatory elements. In human cells, introns with delayed splicing frequently neighbor alternative exons and are associated with RNA-binding factors. Moreover, neighboring introns in human cells tend to be spliced concurrently, implying that splicing occurs cooperatively. Thus, nano-COP unveils the organizational complexity of metazoan RNA processing.