The 21bp repeat element of the human cytomegalovirus major immediate early enhancer is a negative regulator of gene expression in undifferentiated cells.

The major immediate early regulatory region of human cytomegalovirus (HCMV) has a complex set of DNA sites through which both cellular and viral factors coordinately regulate immediate early gene expression. In undifferentiated human teratocarcinoma (T2) cells we have previously shown that major immediate early gene expression is repressed by a differentiation specific nuclear factor MBF1, which binds to the imperfect dyad symmetry located upstream of the enhancer. However, upon differentiation MBF1 decreases resulting in immediate early gene expression. In this study we show, by mobility shift analysis that the same or similar factor(s) also binds to the 21bp repeat of the major immediate early enhancer. Deletion of this 21bp repeat from the immediate early enhancer expression vectors results in increased CAT expression in undifferentiated T2 cells, to levels similar to that in differentiated cells. Consequently, the 21bp repeat of the HCMV enhancer also acts to negatively regulate major immediate early enhancer function in non-permissive cells.     

Promoter-specific function of the TATA element in undifferentiated P19 cells

P19 embryonal carcinoma cells differentiate into neuronal cells when treated with retinoic acid (RA). To explore the importance of core promoter structures in the regulation of gene expression during neuronal differentiation, the activities of three classes of modified or unmodified model promoters (Spec2a, OtxE, and Ars) were compared in P19 cells before and after RA treatment. The Spec2a promoter was activated in undifferentiated cells specifically when the E-box was located at a proximal position, whereas the OtxE promoter was activated when the E-box was in a distal position. The Ars promoter was only slightly activated by this element. In addition, the TATA element reduced the level of activation provided by the E-box, but only when it was located in the Spec2a core promoter. These results indicate that the core promoter structure may govern, at least in part, the stage-specific expression of endogenous genes involved in the neuronal differentiation of P19 cells.