Mature mRNAs accumulated in the nucleus are neither the molecules in transit to the cytoplasm nor constitute a stockpile for gene expression

In higher eukaryotes, the regulation of pre-mRNA processing is still poorly known. The accumulation of various mature mRNAs, which can be observed in the nuclei of mammalian cells, is suggestive of a regulatory role of transp ort. However, the significance of these nuclear mRNA is presently unknown. We have used a tetracycline-regulated promoter to investigate the dynamics of these pools of mRNAs upon arrest of transcription. We observed, for beta -globin and LT-alpha genes, a slow disappearance of these mRNA from the nuc leus, with an apparent half-life that is similar to their cytoplasmic half- life. In view of these dynamics, these mRNA cannot simply be mature mRNAs i n transit to the cytoplasm. They could be mRNAs retained in the nucleus, pr ovided that the regulation of mRNA stability is comparable in the nucleus a nd the cytoplasm. But, because of their limited stability, these nuclear mR NAs cannot constitute a significant stock for gene expression. Alternativel y, they could reflect a bidirectional transport of mRNA, that is, to and fr om the cytoplasm, which would provide a direct explanation for the similari ty in both compartments of their half-life and poly(A) tail shortening over time.