THO is a protein complex that functions in cotranscriptional mRNP formation. Yeast THO1 and SUB2 (Saccharomyces cerevisiae) were identified as multicopy suppressors of the expression defects of the hpr1Delta mutant of THO. Here we show that multicopy THO1 suppresses the mRNA accumulation and export defects and the hyperrecombination phenotype of THO mutants but not those of sub2Delta, thp1Delta, or spt4Delta. Similarly, Sub2 overexpression suppresses the RNA export defect of hpr1Delta. Tho1 is a conserved RNA binding nuclear protein that specifically binds to transcribed chromatin in a THO- and RNA-dependent manner and genetically interacts with the shuttling hnRNP Nab2. The ability of Tho1 to suppress hpr1Delta resides in its C-terminal half, which contains the RNA binding activity and is located after a SAP/SAF (scaffold-associated protein/scaffold-associated factor) domain. Altogether, these results suggest that Tho1 is an hnRNP that, similarly to Sub2, assembles onto the nascent mRNA during transcription and participates in mRNP biogenesis and export. Overexpression of Tho1 or Sub2 may provide alternative ways for mRNP formation and export in the absence of a functional THO complex.
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