The THO complex is involved in transcription, genome stability, and messenger ribonucleoprotein (mRNP) formation, but its precise molecular function remains enigmatic. Under heat shock conditions, THO mutants accumulate large protein-DNA complexes that alter the chromatin density of target genes (heavy chromatin), defining a specific biochemical facet of THO function and a powerful tool of analysis. Here, we show that heavy chromatin distribution is dictated by gene boundaries and that the gene promoter is necessary and sufficient to convey THO sensitivity in these conditions. Single-molecule fluorescence in situ hybridization measurements show that heavy chromatin formation correlates with an unusually high firing pace of the promoter with more than 20 transcription events per minute. Heavy chromatin formation closely follows the modulation of promoter firing and strongly correlates with polymerase occupancy genome wide. We propose that the THO complex is required for tuning the dynamic of gene-nuclear pore association and mRNP release to the same high pace of transcription initiation. Display omitted •Origin and significance of heavy chromatin formation in THO complex mutants•High-resolution genome-wide distribution of heavy chromatin in THO mutants•Single-molecule FISH analysis of HSP104 transcription activation•Mechanistic link between high-frequency firing and heavy chromatin formation The THO complex is conserved and has previously been implicated in multiple processes including transcription and mRNP assembly. Under heat shock conditions, THO mutants accumulate large protein-DNA complexes that alter the chromatin density of target genes (heavy chromatin). Now, Libri and colleagues find that, when genes are transcribed by a high number of polymerases simultaneously, the THO complex is required to coordinate mRNP release with the high pace of transcription initiation. In mutants, mRNP release is delayed, and THO-dependent genes persist at the nuclear pore.