In hematopoietic stem cell development, the expression of critical genes is precisely regulated in a stage specific manner, which supports normal hematopoietic development through adequately regulating timing of cell division, self-renewal, and lineage commitment. Regulation of gene expression is known to take place at least at transcriptional level. In addition to the transcriptional regulation, there are growing evidences that post-transcriptional control of critical genes may play an important role, suggesting an interesting possibility that post-transcriptional control may also play a role in hematopoiesis. Here, we provide the evidence that the expression of Notch1, a key factor in lymphoid development, is controlled at post-transcriptional level in hematopoietic stem cell (HSC). By quantitative PCR, Notch1 mRNA is substantially expressed at HSCs as well as common lymphoid progenitors (CLPs) or double negative (DN) thymocytes. However, Notch1 protein is detected at very low level in HSCs compared to CLPs or DN thymocytes, suggesting that Notch1 expression is regulated at post-transcriptional level in HSC. To investigate the effect of 3′UTR (untranslated region) on post-transcriptional regulation, we prepared a retrovirus sensor vector, in which 3′UTR of target gene is placed between the GFP coding region and the retrovirus 3′LTR, and found that induction of the sensor vector with the 3′UTR sequence of Notch1 showed marked suppression of the GFP intensity at the HSC stage. This effect was not observed when we introduced the vector into DN thymocytes. Suppression of Notch1 by its 3′UTR was further confirmed by using a retrovirus vector which has two distinct markers of YFP and GFP-3′UTR fusion genes under bi-directional EF1 promoter. Deletion mutant analysis showed that the responsible region required for this post-transcriptional suppression is confined to 120-bp sequence within Notch1 3′UTR so far. These data suggest that the expression of Notch1 should be regulated at post-transcriptional level by its 3′UTR at the HSC stage and our data provide the first evidence that the stage-specific translational regulation can play an important role in organization of hematopoietic development.

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