Find the missing character?

Starting with the letters in segments of the top left circle, add 3 to the numerical values of the letters, and write the results in the top right circle, with the relative positions of the letters moving 1/4 turn clockwise. Repeat this pattern, adding 4 to the values of the letters for the lower right hand circle, and adding 5 for the letters in the lower left hand circle, putting the results in the segments 1/4 turn clockwise each time.