Protein synthetic patterns during the asexual life cycle of Volvox carteri

Abstract

The polypeptide labeling patterns of somatic cells, gonidia (asexual reproductive cells), embryos, and juvenile spheroids of Volvox carteri cultures synchronized by a light/dark cycle were studied as a function of developmental stage and incubation condition. Specimens were exposed to 35SO = 4 for 1-hr periods at selected intervals throughout the asexual life cycle; proteins were then extracted and analyzed by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by fluorography. Although sulfation accounts for more than half the 35S incorporated, the conditions of extraction and electrophoresis employed resulted in exclusion of most sulfated products and inclusion of nearly all products bearing incorporated sulfur amino acids in the electrophoretic analysis. Hence SDS-PAGE profiles reflect relative rates of synthesis of major polypeptides. The first phase of these studies involved examination of stage-specific differences in protein synthetic patterns. Because a single developmental stage exhibits different protein synthetic patterns in light and darkness, detailed developmental comparisons were made only on organisms or cells exposed to label in the light. They yielded the following results: Shortly after the completion of embryogenesis (while all cells are still linked by numerous cytoplasmic bridges) presumptive somatic cells and gonidia exhibit a nearly identical pattern of labeling of the major polypeptides. In just a few hours, however, as cytoplasmic bridges begin to break down, the synthetic patterns of the two cell types begin to diverge; with passing time this divergence becomes progressively greater. By the time gonidia are mature, the patterns of labeling of major polypeptides by somatic cells and gonidia exhibit far more differences than similarities. Embryos derived from these mature gonidia then exhibit numerous, reproducible, stage-specific changes in polypeptide labeling throughout embryogenesis. However, two glycoproteins that previous authors implicated in the control of the differentiative cleavage division are here shown to be labeled in the parental somatic cells, not in the embryos as was previously supposed; hence a central role for them in embryonic development seems highly unlikely. In the second phase of this study the effects of light on protein synthetic patterns of organisms at selected developmental stages were analyzed. At all stages marked, rapid, reversible changes in the pattern of labeling of major polypeptides occur when cultures are transferred from light to dark or vice versa, but these changes are most marked in juvenile spheroids at the end of the dark period during which they had completed their embryogenesis. Some, but by no means all, of the changes induced by light can be attributed to stimulated synthesis of chloroplast proteins, on both chloroplast and cytosol ribosomes. The proteins made at the beginning of one light period are not identical to those made at the end of the preceding light period. We postulate that it is the selective activation, by light, of protein synthesis on transcripts accumulated during the dark period that accounts for the fact that the life cycle can be synchronized by light.