Heat oven to 350°F. Spray 15x10x1-inch baking pan with nonstick cooking spray.
In large bowl, combine sugar and 1/2 cup margarine; beat until light and fluffy. Add molasses and egg; beat well.
In small bowl, combine all-purpose flour, whole wheat flour, baking soda and ginger; blend well. Add to sugar mixture alternately with buttermilk; mix well. Stir in 2 teaspoons orange peel. Spread evenly in sprayed pan.
Bake at 350°F. for 15 to 20 minutes or until toothpick inserted in center comes out clean. Cool 30 minutes or until completely cooled.
In small bowl, combine all frosting ingredients, adding enough orange juice for desired spreading consistency; beat until smooth. Spread over cooled bars. Cut into bars.