Catalysis of the reaction: S-adenosyl-L-methionine + dihydroflavodoxin + -glycine = 5'-deoxyadenosine + L-methionine + flavodoxin semiquinone + -glycin-2-yl radical.
PFL-glycine:S-adenosyl-L-methionine H transferase (flavodoxin-oxidizing, S-adenosyl-L-methionine-cleaving) activity