Regulation of protein synthesis in the plasmodial phase of Physarum polycephalum

1. The rate of protein synthesis changes very little during the first 2-3 h (S phase) of the nuclear division cycle in plasmodia of Physarum polycephalum and then increases continuously during G2 phase, so that by the end of the cycle the rate has doubled relative to that in S phase. Protein synthesis appears to continue during mitosis. 2. Fractionation of extracts of plasmodia, labeled with [3H]lysine for 1 h, by two-dimensional electrophoresis indicated that most if not all proteins are synthesised throughout the nuclear division cycle. However, two metabolically stable polypeptides, the synthesis of which occurs predominantly in Gs phase, were detected. 3. Using a double-labelling procedure, the differential rates of synthesis of 30 relatively abundant polypeptides were measured in relation to the nuclear division cycle. As a group, their differential rates of synthesis increase during the cycle so that their actual rates of synthesis increase 4-6-fold. This implies that their synthesis is regulated over and above any simple change due to a doubling in the number of genes during S phase.