ABSTRACT
Pre-mRNA splicing is tightly coordinated with transcription in yeasts, and introns can be removed soon after they emerge from RNA polymerase II (Pol II). To determine if splicing is similarly rapid and efficient in mammalian cells, we performed long read sequencing of nascent RNA during mouse erythropoiesis. Remarkably, 50% of splicing occurred while Pol II was within 150 nucleotides of 3′ splice sites. PRO-seq revealed that Pol II does not pause around splice sites, confirming that mammalian and yeast spliceosomes can act equally rapidly. Two exceptions were observed. First, several hundred introns displayed abundant splicing intermediates, suggesting that the spliceosome can stall after the first catalytic step. Second, some genes – notably globins – displayed poor splicing coupled to readthrough transcription. Remarkably, a patient-derived mutation in β-globin that causes thalassemia improves splicing efficiency and proper termination, revealing co-transcriptional splicing efficiency is a determinant of productive gene output.