Effects of gene dosage on protein synthesis rate in Paramecium tetraurelia: Implications for regulation of cell mass, DNA content and the cell cycle

Abstract

Both cell mass and DNA content can vary considerably in Paramecium. This study examines protein and RNA synthesis rates in cells with various DNA contents. At lower DNA contents the protein synthesis rate is proportional to DNA content. At a characteristic ratio of gene dosage to cell mass, the protein synthesis rate levels off. This plateau rate of protein synthesis is strongly correlated with cell mass throughout the cell cycle. These results are consistent with two alternative interpretations. The results suggest that the protein synthesis rate is template-limited at low DNA contents and then levels off, either because the protein-synthesizing system is saturated with gene transcripts at high DNA contents, or because of a concentration-dependent feedback system which regulates the transcript concentration in the cytoplasm. A simple deterministic cell cycle simultation model shows that the saturation hypothesis is consistent with the observed effects of experimental increase in either DNA or protein content on the subsequent DNA and protein content and cell cycle length.