TUPELO - A Mississippi martial arts instructor has pleaded not guilty to charges he sent ricin-laced letters to President Barack Obama and two other public officials.James Everett Dutschke, 41, who was arrested on April 27, pleaded not guilty to all five counts of a grand jury indictment, court records filed on Tuesday showed.His trial has been set for July 29 and Dutschke faces a maximum penalty of life in prison for threatening the president and others by mail and "possessing a biological agent, toxin and delivery system for use as a weapon."