AT1 receptor expression is mainly regulated posttranscriptionally involving modulation of RNA stability which is dependent on protein binding to the cognate sequence bases 2179-2195 within the 3' untranslated region of the AT1 receptor RNA. This region contains an AUUUUA hexamer which forms part of a stem-loop structure. To clarify the significance of the AUUUUA hexamer for AT1 receptor mRNA regulation, mutations were introduced inside, up- or downstream of it. In vitro decay assays, transfection experiments, and UV-light mRNA protein crosslink assays could demonstrate that mutations within the AUUUUA hexamer disrupted AT1 receptor mRNA degradation as well as the binding of polysomal proteins. In contrast, modification in the neighboring sequence had no effect on mRNA turnover or protein, binding. Computer modelling revealed that the AUUUUA hexamer is important for the formation of a stem-loop structure which in turn is relevant for mRNA-protein interactions. These findings indicate that the AUUUUA hexamer is essential for the posttranscriptional modulation of the AT1 receptor mRNA expression. (c) 2005 Published by Elsevier Inc.