Several nuclear pore-associated factors, including the SUMO-protease Ulp1, have 
been proposed to prevent the export of intron-containing messenger 
ribonucleoparticles (mRNPs) in yeast. However, the molecular mechanisms of this 
nuclear pore-dependent mRNA quality control, including the sumoylated targets of 
Ulp1, have remained unidentified. Here, we demonstrate that the apparent 
'pre-mRNA leakage' phenotype arising upon ULP1 inactivation is shared by 
sumoylation mutants of the THO complex, an early mRNP biogenesis factor. 
Importantly, we establish that alteration of THO complex activity differentially 
impairs the expression of intronless and intron-containing reporter genes, 
rather than triggering bona fide 'pre-mRNA leakage'. Indeed, we show that the 
presence of introns within THO target genes attenuates the effect of THO 
inactivation on their transcription. Epistasis analyses further clarify that 
different nuclear pore components influence intron-containing gene expression at 
distinct stages. Ulp1, whose maintenance at nuclear pores depends on the Nup84 
complex, impacts on THO-dependent gene expression, whereas the nuclear 
basket-associated Mlp1/Pml39 proteins prevent pre-mRNA export at a later stage, 
contributing to mRNA quality control. Our study thus highlights the multiplicity 
of mechanisms by which nuclear pores contribute to gene expression, and further 
provides the first evidence that intronic sequences can alleviate early mRNP 
biogenesis defects.