LOS ANGELES (Reuters) - R&B singer Chris Brown pleaded not guilty on Tuesday to a hit-and-run charge in a case that could lead to jail time for the Grammy-winning performer, a Los Angeles Superior Court spokeswoman said. Brown had been on probation for assault, stemming from the 2009 beating of his then-girlfriend and fellow R&B star Rihanna, when he was involved in a Los Angeles traffic accident on May 21. He allegedly rear-ended another vehicle and fled the scene. Lesser charges of driving without a valid license and driving without proof of insurance were dismissed. ...