A runner in Auburn has sustained life-threatening injuries after he was struck Thursday afternoon by a motorcycle.Auburn police said the motorcycle, driven by 27-year-old Mason Perez, of Auburn, was coming over a hill on South Witham Road when he hit 52-year-old Tyrone Fulgham, of Auburn, as he was jogging.Fulgham sustained what police described as life-threatening injuries. He was transported to Central Maine Medical Center and was last listed in serious condition.Perez was not seriously injured in the crash.Police said witnesses reported the motorcycle was speeding at the time of the crash. Their investigation continues.So far, Auburn Police have charged Perez with operating without a license.