The overall goals of the proposed research are (1) to identify those proteins which are necessary for nuclear DNA replication and whose synthesis is inhibited by cycloheximide in Physarum polycephalum, and (2) to determine the function of such proteins in the replication process. Results to date indicate that the "cycloheximide-sensitive" proteins are required for the elongation of progeny strands within DNA replication units. The deficiency in the nuclei of cycloheximide-treated cultures is reflected by a depression of the activity of a DNA polymerase present in such nuclei. The immediate objectives are (a) to determine whether the protein deficiency in nuclei isolated from cultures pre-treated with cycloheximide can be used in a complementation assay for the required proteins; (b) to attempt to isolate mutants of Physarum with conditional defects in DNA replication; and (c) to determine the effect of cycloheximide on the synthesis of putative RNA primers for eukaryotic DNA replication.