Chemiluminescence in the system xanthine oxidase + xanthine + 10,10'-dimethyl-9,9 '-biacridylium nitrate (DBA + +) is a consequence of the two-electron reduction of DBA + + and the one-electron reduction of 02 by the reduced enzyme. The reduced DBA and oxygen radical, so formed, react to form an excited DBA + + derivative which emits light as it returns to the ground state.

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Strains of E. coli bearing a temperaturesensitive mutation at the dnaE locus were unable to support growth of bacteriophage XX-174 at 43°while permitting normal growth at 300, The dnaE gene product was implicated in all stages of OX-174 DNA replication. Differences were observed between a double mutant (pol A-, dnaEts) and a single mutant (pol A +, dnaEts) in the ability to synthesize parental replicating form and its subsequent replication. The implications of these observations are discussed in terms of the relationship between DNA polymerases-I and -III.Replication of bacteriophage 4X-174 DNA is accomplished in three temporally and mechanistically distinct stages. The first stage, formation of a double-stranded "replicating form (RF)" by synthesis of a complementary strand of the infecting single strand of the virus, is accomplished by hostcell enzymes (1). The second stage, semi-conservative RF replication, requires at least one phage function and an undetermined number of host functions. The third stage, conservative displacement synthesis of progeny viral strands from a RF precursor, is also dependent on both phage and host functions (2).There are three presently known DNA polymerases of Escherichia coli (3). Mutants defective in either polymerase-I (4) or -II (5, 6), or both, are able to support the growth of OX-174. (pH 7.5). After autoclaving, 10 ml of 0.1 M MgCl2 and 10 ml of 0.25 M CaCl2 were added per liter. 0.06 ml of MeMS per 100 ml of L-Tris agar was added to plates after pouring and the plates were used the same day.Transductions. The method of Lennox (10) was used for transduction. Recipient cells were grown to a density of 2 X 108 cells per ml and infected with a multiplicity of 3.0.Infections and Preparation of DNA. Cells were grown at 300 to a density of 1-2 X( 108/ml, transferred to 43°, incubated for 20 min, and infected with 32P-labeled, 5-bromouracil-substituted 4Xam3 at a multiplicity of 1.0. Samples were rapidly chilled by dilution in -70°acetone, and the cells were collected by centrifugation. In the replication experiment (Fig. 2), washing, lysis, and removal of E. coli DNA were performed as described by Franke and Ray (11)

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