Dissolve yeast in warm water in a large mixing bowl. Add buttermilk, eggs, 2 1/2 cups flour, 1/2 cup margarine, 1/2 cup white sugar, baking powder and salt. Blend 30 seconds with mixer on low speed, scraping sides and bottom of bowl. Beat 2 minutes on medium speed.
Stir in remaining 3 cups flour. Dough should remain soft and slightly sticky. Knead for 5 minutes, or about 200 turns on a lightly floured board.
Form dough into a round and place it in an oiled bowl. Cover it with a damp cloth and leave it in a warm place to rise until doubled.
Divide dough in half and roll each half into a 12x7 inch rectangle. Spread each half with 1 tablespoon soft margarine, and sprinkle with 1/4 cup white sugar and 1 teaspoon cinnamon. Roll up halves, beginning at wide side. Seal well by pinching the seams. Cut each roll into 12 slices.
Coat two 9 inch round cake pans with 1/4 cup melted butter, 1/4 cup brown sugar, and 1/2 cup chopped pecans in each pan. Place 12 dough slices in each pan, leaving a small space between slices. Let rise until doubled.
Bake in a preheated 375 degrees F (190 degrees C) oven for about 30 minutes. Invert pans onto serving plate.