Abstract
RF I DNA of phage fd containing 5-bromo-deoxyuridine (br5Ud) or deoxyuridine (Ud) instead of deoxythymidine (Td) inthe codogenic strand was synthesized in vitro. The modified genomes could be cleaved by restriction endonuclease Hpa II. Although the recognition site of Hpa II is CCGG, the cleavage rate was significantly reduced with Ud-containing DNA. Both base substitutions altered the mobilities of several DNA fragments under the conditions of polyacrylamide gel electrophoresis. The fragments containing binding sites for RNA polymerase were assayed for the rates of stable complex formation. The substitution of Td for both, Ud and br5Ud, strongly influenced this parameter. Thus the methyl group of Td has to be regarded as one of the sites in DNA which determine the rate of stable RNA polymerase binding and thereby possibly mediate promoter activity in vitro (24,25,26). In most cases the rate of complex formation was decreased by Ud, but increased by br5Ud.