Preheat oven to 450 degrees. Sift together flour, salt, baking powder, baking soda and sugar. Cut in shortening with a pastry blender or fork. Dissolve yeast in warm water. Add slightly cooled yeast to buttermilk and blend into dry ingredients. Form into a ball and place on a lightly floured board. Roll out the dough until thin and cut into small biscuits. Brush biscuits with melted butter and place on a cookie sheet in stacks of two. Let biscuits rise until doubled. Bake for 12 minutes. Serve warm. Yields 3 dozen.