A San Francisco teen charged in the robbery and beating of an 18-year-old actor on a San Francisco Municipal Railway bus two weeks ago pleaded not guilty to the charges in court this morning and was ordered held on $200,000 bail.Uluao Mase, also 18, and two juveniles were arrested last week in connection with the assault on Christopher Borgzinner on a 9-San Bruno bus at about 5:45 p.m. on Oct. 5.Police credited the arrests to surveillance footage taken from a video camera on the bus. A fourth suspect is still being sought.The attack left Borgzinner with a fractured orbital socket, cheekbone and jaw, according to prosecutors.Police and prosecutors said Mase confronted Borgzinner on the bus and asked to use his cell phone. Mase and the other suspects then asked if Borgzinner belonged to a gang before assaulting him and stealing his iPod and wallet. Borgzinner, a San Francisco resident, has no gang affiliation, police said. He plays “Nacho” in the upcoming film “La Mission,” starring Benjamin Bratt and directed by Bratt’s brother Peter Bratt. The film was shot in San Francisco’s Mission District.Mase pleaded not guilty at his arraignment today to charges of robbery, assault and possession of an assault weapon, a gun that was found in his backpack but was apparently not used in the alleged crime.He remains in custody and his preliminary hearing in the case is scheduled for Nov. 2.