In an action against the Commission instituted in a district court of the United States, service of the summons and of the complaint upon the Commission shall be made by delivering a copy thereof to the United States attorney for the district in which the action is brought, or to an assistant United States attorney, or to a clerical employee designated by the United States attorney to accept service in a writing filed with the clerk of the court, and by sending a copy of the summons and of the complaint to the Commission by registered or certified mail.