Dissolve yeast in warm water in a small bowl; let stand 5 minutes.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (flour through salt) in a large bowl. Cut in shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Add yeast mixture and buttermilk; stir just until moist. Cover and chill 1 hour.
Preheat oven to 450°.
Turn dough out onto a heavily floured surface; knead lightly 5 times. Roll dough to a 1/2-inch thickness; cut with a 3-inch biscuit cutter. Place on a baking sheet coated with cooking spray. Brush melted margarine over biscuit tops. Bake at 450° for 13 minutes or until golden.