Reagents. Cesium chloride (DNA optical grade) was obtained from Gallard-Schlesinger Chemical Co., Carle Place, N.Y. All enzymes were supplied by Boehringer Mannheim Biochemicals, Indianapolis, Ind., and Bethesda Research Laboratories, Gaithersburg, Md., and were used according to the specifications of the manufacturer. Radiolabeled deoxyribonucleotides were supplied by Amersham Corp., Arlington Heights, Ill., and New England Nuclear Corp., Boston, Mass. All other reagents and chemicals were supplied by Fisher Scientific Co.