Bacillus subtilis W23 was infected with a clear-plaque variant of SP-10 phage, namely, SP-lOc. Exogenous thymidine was not incorporated into phage DNA (even in the presence of deoxyadenosine), nor was there any transfer of thymidine nucleotides from bacterial to viral DNA. The lytic program was unaffected by concentrations of 5-fluorodeoxyuridine sufficient to reduce bacterial DNA synthesis by >95%. Although these data are consistent with the interpretation that thymidine nucleotides are excluded from phage DNA, formic acid digests of SP-lOc DNA contained what appeared to be the four conventional bases; however, adenine and thymine were not recovered in equimolar yields. DNA-RNA hybridization and hybridization competition experiments were done. Synthesis of host RNA started to wane moments postinfection and stopped completely by 36 min. SP-lOc coded for discrete classes ofearly and late RNA. The possibility of discrete subclasses of early RNA exists. Replication of the bacterial genome appeared to terminate 12 min postinfection. Degradation of the host DNA to acid-soluble material started at 36 min and, by the end of the latent period, >90% of the host chromosome was hydrolyzed. Four apparent phage-coded enzymes have been identified. A di-and triphosphatase degraded dUTP, dUDP, dTTP, and dTDP (and, to a lesser extent, dCDP and dCTP) to the corresponding monophosphates; the enzyme had no apparent activity on dATP and dGTP. SP-lOc also coded for a DNA-dependent DNA polymerase, lysozyme, and a nuclease that degrades native bacterial DNA. Judging from the dependence of enzyme synthesis on the time of addition of rifampin (an inhibitor of the initiation of RNA synthesis), messengers for the di-and triphosphatase, as well as the nuclease, are transcribed from promoters that start to function 6 min postinfection. Promoters for polymerase and lysozyme did not become functional until 8 and 16 min postinfection, respectively. MATERIALS AND METHODS Phage and bacteria. B. subtilis W23 (ATCC 23059) was used as host. SP-lOc is a clear-plaque variant present in ATCC 23059B. SP-lOc was readily inactivated by antiserum prepared against SP-10c and provided by K. Bott (Table 1). The eclipse and latent periods of SP-10c at 37°C were 25 and 55 min, 84