Abstract
Coupling between transcription and RNA processing is key for gene regulation. Using live-cell photobleaching techniques, weinvestigated the factor TCERG1, which coordinates transcriptional elongation with splicing. We demonstrate that TCERG1 ishighly mobile in the nucleoplasm and that this mobility is slightly decreased when it is associated with speckles. Dichloro-1-βD-ribofuranosylbenzimidazole (DRB) but not α-amanitin treatment reduced the mobility of TCERG1, which suggestsinteraction with paused transcription elongation complexes. We found that TCERG1 mobility is rapid at the transcription site(TS) of a reporter that splices post-transcriptionally and that TCERG1 is recruited to the active TS independent of the CTD ofRNAPII, thus excluding phosphorylated CTD as a requirement for recruiting this factor to the TS. Importantly, the mobility ofTCERG1 is reduced when the reporter splices cotranscriptionally, which suggests that TCERG1 forms new macromolecularcomplexes when splicing occurs cotranscriptionally. In this condition, spliceostatin A has no effect, indicating that TCERG1rapidly binds and dissociates from stalled spliceosomal complexes and that the mobility properties of TCERG1 do not dependon events occurring after the initial spliceosome formation. Taken together, these data suggest that TCERG1 bindsindependently to elongation and splicing complexes, thus performing their coupling by transient interactions rather than bystable association with one or the other complexes. This finding has conceptual implications for understanding the couplingbetween transcription and RNA processing.