Coupling of co-transcriptional splicing and 3’ end Pol II pause during termination in Arabidopsis

In Arabidopsis, RNA Polymerase II (Pol II) often pauses within a few hundred base pairs downstream of the polyadenylation site, reflecting efficient transcriptional termination, but how such pause is regulated remains largely elusive. Here, we studied Pol II dynamics at gene 3’ end by combining comprehensive experiments with mathematical modelling. We generated high-resolution Ser2P Pol II positioning data specifically enriched at gene 3’ end and defined a 3’ end pause index (3’PI). The position but not the extent of the 3’ end pause correlates with termination window size. The 3’PI is not decreased but even mildly increased in the termination deficient mutant xrn3, indicating 3’ end pause is a regulatory step early during the termination and before XRN3 mediated RNA decay that releases the Pol II. Unexpectedly, 3’PI is closely associated with gene exon numbers and co-transcriptional splicing efficiency. Multiple exons genes often display stronger 3’ end pauses and more efficient on-chromatin splicing than genes with few exons. Chemical inhibition of splicing strongly reduces the 3’PI and disrupts its correlation with exon numbers but does not globally impact 3’ end readthrough levels. These results were further confirmed by fitting Pol II positioning data with a mathematical model, which enables the estimation of parameters that define Pol II dynamics. Our work highlights that the number of exons via co-transcriptional splicing is a major determinant of Pol II pausing levels at gene 3’ end in plants. pNET-seq was performed using Pol II Ser 2P antibody 3E10 to profile the elongating transcripts genome wide in Arabidopsis. As a complementary, RNA-seq was performed in the same conditions to measure the nascent RNA.