Phosphorylation of ribosomal proteins during the cell cycle of Physarum polycephalum

Physarum polycephalum has been used as a model system to study the phosphorylation of ribosomal proteins during the cell cycle. The results showed that the phosphate content of S3, the major ribosomal phosphoprotein in this organism, was constant during all phases of the cell cycle. No additional ribosomal phosphoproteins were observed. These results differ significantly from those reported earlier by Rupp, R.G., Humphrey, R.M. and Shaeffer, J.R. (Biochim. Biophys. Acta (1976) 418, 81-92) and suggest that the use of thymidine or hydroxyurea to synchronize cell population may affect the phosphorylation of ribosomal proteins. The results are discussed in relation to protein synthesis and cAMP level during the cell cycle.