Seth Johnson, 29, of Frankford Avenue near Unity Street, was shot once in the back about 11:50 p.m. last night on Church Street near Penn, police spokesman Sgt. Ray Evers said. He was conscious and talking when police arrived, and told them a bearded black man on a bike shot him, Evers said. But he died at 12:44 a.m. at Temple University Hospital. The motive is unknown, and police have no suspects. Tipsters can call homicide detectives at (215) 686-3334 or -3335.