Abstract
Ten temperature-sensitive mutants of T4D have been examined to find the times in the lytic cycle at which a shift to a restrictive temperature no longer exerts a lethal effect. Nine of these mutants play a role in phage DNA synthesis. The results of these experiments suggest that the products of three of the genes tested-genes 42, 44, and 45-have control or complexing functions in addition to any enzymatic ones. The data also indicate that the products of genes 30 and 41 may act cooperatively. The properties of mutants of gene 43 (DNA polymerase) and gene 56 (dCTP-dUTPase) suggest that the production of a certain length of concatameric DNA is necessary before viable phage can be produced.