Combine 1/2 cup warm water, yeast and 1 tablespoon sugar in large glass measuring cup and stir until yeast dissolves. Let yeast mixture stand at room temperature until foamy, about 10 minutes.

In large bowl of heavy-duty mixer fitted with whisk attachment, beat 5 eggs until blended. Add oil, salt and 3/4 cup sugar and beat until pale yellow and slightly thickened, about 4 minutes. Beat in 2/3 cup warm water. Add yeast mixture and beat until blended. Remove whisk and fit mixer with dough hook. Add enough flour 1 cup at a time to form smooth dough, beating well after each addition. Beat on medium speed until smooth and elastic, about 5 minutes, adding flour by tablespoonfuls if sticky. Turn out onto floured surface and knead 2 minutes.

Lightly oil large bowl. Add dough, turning to coat with oil. Cover with plastic wrap, then with clean kitchen towel. Let dough rise in warm draft-free area until doubled in volume, about 1 hour.

Punch down dough. Cover with plastic and clean kitchen towel and let rise 30 minutes.

Grease 2 large baking sheets. Turn out dough onto lightly floured surface. Divide dough into 2 equal portions. Divide each portion into 3 equal pieces. Roll each piece into 9-inch-long rope. Braid 3 ropes together; pinch ends together to seal. Repeat with remaining dough pieces, forming 2 braids. Place each braid on baking sheet. Cover with towel . Let rise in warm area until almost doubled, about 30 minutes.

Preheat oven to 400F. Whisk yolk with 1 tablespoon water to blend. Brush dough with egg mixture. Bake 10 minutes. Reduce oven temperature to 350F. Bake until bread is golden brown and sounds hollow when tapped on bottom, about 35 minutes. Transfer loaves to rack and cool completely. (Can be prepared 1 day ahead. Wrap tightly in plastic and store at room temperature.)

Per serving: 5316 Calories (kcal); 201g Total Fat; (34% calories from fat); 136g Protein; 727g Carbohydrate; 1148mg Cholesterol; 2447mg Sodium Food Exchanges: 47 1/2 Grain(Starch); 5 1/2 Lean Meat; 0 Vegetable; 0 Fruit; 35 1/2 Fat; 0 Other Carbohydrates