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The transient expression of many different genes is mediated by the inducible transcription factor p50‐p65 NF kappa B, which in turn is regulated by complex formation with its inhibitor I kappa B alpha. We describe here that in porcine aortic endothelial cells, either IL‐1 alpha, TNF alpha or LPS upregulates an inhibitor of NF kappa B which we refer to as ECI‐6. ECI‐6 is by structural and functional criteria an I kappa B alpha protein, the porcine homologue of MAD‐3, pp40 and RL/IF‐1. We have studied the promoter of the ECI‐6/I kappa B alpha gene and provide three lines of evidence that its expression is directly regulated by NF kappa B. First, the 5′ regulatory region of ECI‐6/I kappa B alpha contains two sites that bind NF kappa B in electrophoretic mobility shift assays. Second, expression following transfection of an ECI‐6/I kappa B alpha promoter‐luciferase reporter construct is dependent on a co‐transfected …