In a large mixing bowl stir together 2 cups of the flour and the yeast; set aside. In a medium saucepan heat and stir water, sugar, margarine, and salt just until warm (120 to 130 degrees) and margarine almost melts. Add water mixture to dry mixture along with the eggs. Beat with an electric mixer on low to medium speed for 30 seconds, scraping the sides of the bowl constantly. Beat on high speed for 3 minutes. Using a wooden spoon, stir in as much of the remaining flour as you can.

Turn dough out onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately stiff dough that is smooth and elastic (6 to 8 minutes total). Shape the dough into a ball. Place in a lightly greased bowl, turning once to grease surface of the dough. Cover and let rise in a warm place until double in size (about 1 hour).

Punch dough down; divide in thirds. Cover; let rest 10 minutes. Roll each third into an 18-inch rope. Place ropes on a large baking sheet 1 inch apart and braid. Cover; let rise 30 minutes or until nearly double. Brush braid with egg yolk and sprinkle with poppy seed, if desired.

Bake in a 375 degree F oven for 25 to 30 minutes or until bread sounds hollow when you tap the top (if necessary to prevent overbrowning, cover loosely with foil the last 10 minutes of baking). Remove bread from pan. Cool on wire rack. Makes 1 braid (32 servings).

Wrap cooled bread in freezer wrap or place in airtight freezer container. Freeze up to 1 month. Thaw overnight. Wrap bread in foil; bake in 375 degree F. oven for 10 minutes or until warm.

If you don't want to make a braid, shape this dough into two plain loaves and place in two greased 8x4x2-inch loaf pans. Let rise and bake as directed.