For crust, process crackers in food processor to fine crumbs. Measure 1.25 cups. Mix crumbs with sugar. Add oil and mix well. Lightly grease 9-inch springform pan. Press crumb mixture in even layer on bottom and about 1 inch up sides of the pan. Bake at 350 degrees for 8 minutes. Cool completely. Leave oven at 350 degrees.

For filling, beat cheese with non-fat sour cream at low speed until very smooth. Gradually beat in sugar. Beat in flour, egg yolks, lemon peel and vanilla.

Whip egg whites in small bowl until stiff. Fold them into cheese mixture. Carefully pour filling into cooled crust. Bake until top center is firm (cracks will form in cake) about 50 minutes. Remove from oven. Cool 15 minutes. Increase oven temperature to 425 degrees.

to prepare topping, pour off any liquid from top of yogurt. Mix yogurt with nonfat sour cream, sugar and vanilla. Spoon topping evenly over cake. Carefully spread topping in even layer, without letting it drip over edge of cake. Return cake to oven and bake 7 minutes longer. Cool to room temperature. Cover and refrigerate at least 2 hours before serving. Remove sides of springform pan just before serving. Makes 8 to 10 servings.