Human THO coordinates transcription termination and subsequent transcript release from the HSP70 locus

A multiprotein complex, THO/TREX, couples the transcription, 3'-end formation and nuclear export of mRNAs. In this study, we report that crucial factors for mRNA processing, such as XRN2, DDX5/DDX17 and CstF64, are copurified with human THO (hTHO). Using chromatin immunoprecipitation, we found increased cross-linking of XRN2 and CstF64 to the RNA polymerase II (RNAP II) pause site of the HSPA1A gene upon down-regulation of THOC5, a metazoan-specific component of hTHO. As observed in THOC5-depleted cells, knockdown of XRN2 blocked HSP70 transcript release and increased the amount of CstF64 at the pause site. In addition, our data indicate that DDX5/DDX17 is also required for HSP70 transcript release. As the degradation of read-through transcripts, but not cleavage at polyadenylation sites per se, was hindered upon THOC5 or DDX5/DDX17 down-regulation, these factors appear to influence transcriptional termination. Interestingly, over-expression of RNase H suppressed the accumulation of HSP70 transcripts in nuclear foci in THOC5- or DDX5/DDX17-depleted cells. Thus, we propose a model in which hTHO, along with DDX5/DDX17, restricts the formation of R-loops, thereby facilitating the XRN2-mediated transcriptional termination and release of the mature transcript from the HSPA1A locus.