Changes in cytoplasmic DNA-binding phosphoproteins during the cell cycle of Physarum polycephalum

The cytoplasmic DNA-binding proteins of Physarum polycephalum were examined during two successive cell cycles for alterations in the phosphorylated species. Plasmodia were grown continuously in [³H]leucine and [³²P]orthophosphate in order to label proteins with ³H and phosphoproteins additionally with ³²P. Cytosol proteins were passed through columns of double-and single-stranded DNA cellulose. Proteins with affinity for DNA-cellulose were eluted with increasing NaCl concentrations and were examined by polyacrylamide gel electrophoresis and gel autoradiography. A general decrease in phosphorus content of approx. 30% was found in both DNA-binding and non-binding proteins during the G2 phase of the cell cycle. When compared among the cytoplasmic DNA-binding proteins this decreased phosphorus content was attributable to the attenuation or disappearance of several phosphorylated DNA-binding proteins. In addition, several differences in the polypeptide profiles of phosphorylated and non-phosphorylated DNA-binding proteins were found at the same point in two successive cell cycles. In general, there was a decreased phosphate content among the DNA-binding proteins during the second successive cell cycle. It is suggested that this decrease is related to a depletion of nutrients, and that the coordinated alterations in the phosphate content of most cytoplasmic DNA-binding proteins is indicative of their functional relatedness.