Component the THO subcomplex of the TREX complex, which operates in coupling transcription elongation to mRNA export. The THO complex is recruited to transcribed genes and moves along the gene with the elongating polymerase during transcription. THO is important for stabilizing nascent RNA in the RNA polymerase II elongation complex by preventing formation of DNA:RNA hybrids behind the elongating polymerase. It functions in cotranscriptional formation of an export-competent messenger ribonucleoprotein particle (mRNP) by facilitating the loading of ATP-dependent RNA helicase SUB2 and the mRNA export factor YRA1 along the nascent mRNA