Grease large ovenproof glass bowl and set aside. Melt margarine with 1 cup water in small saucepan over low heat.

Let cool to lukewarm. Place 2 cups flour in large bowl of electric mixer and make a well in center. Combine sugar, yeast and salt in well. Add melted margarine mixture. Using dough hook, blend at low speed until thoroughly combined, 2 to 3 minutes.

Add 4 eggs and additional egg yolk, one at a time, blending well at medium low speed after each addition. Continue mixing until dough resembles thick batter, about 7 minutes.

Blend in 3 cups flour, 1/4 cup at a time, mixing at low speed until dough is heavy and sticky and pulls away from sides of bowl, about 7 minutes.

Sprinkle remaining 1/2 cup flour on work surface and knead until smooth and elastic, about 5 minutes. (Dough will be sticky.)

Transfer dough to prepared ovenproof glass bowl, turning to coat entire surface. Pour boiling water into medium ovenproof glass bowl. Set bowl with dough atop; do not allow bottom of large bowl to touch boiling water. Cover dough with cloth and let rise in warm draft-free area until doubled in volume, 40-50 minutes.

Grease 2 baking sheets, or two 9x5 inch loaf pans, or 1 of each. Divide dough in half. Separate each half into thirds. Rollinto ropes 12 to 13 inches long. Make 2 braids with ropes, tucking ends under.

Transfer loaves to prepared pans. Pour boiling water into 2 medium ovenproof glass bowls. Arrange pans with loaves over water. Cover; let rise in warm draft-free area until doubled, about 30 minutes.

Preheat oven to 400º. Remove loaves from over water. Bake until golden, about 15 minutes. Combine egg white and 1/2 teaspoon water; brush over loaves. Continue baking until browned, 8 to 10 minutes.

Turn loaves out onto racks and cool slightly. Serve warm or allow to cool, wrap with plastic and store at room temperature.