Studies on the nucleic acid associated proteins of the cellular slime mould Dictyostelium discoideum.

The work presented in this thesis has utilized the advantages of a microbial system of development in order to determine possible ways in which nuclear and cytoplasmic controls may regulate cellular differentiation. The studies that have been carried out have been concerned with the structure of the basic nuclear proteins and ribosomes of Dictyostelium discoideum. If the process of development is, in part, regulated by functional changes in these components, then it is reasonable to expect to find structural changes in these components when isolated from cells in different developmental stages. The differences between the basic nuclear protein content of vegetative and developing cells (as determined by urea-polyacrylamide gel electrophoresis) were very slight. No stage-specific basic nuclear proteins were observed, and the only reproducible difference in the electrophoretic patterns was a quantitative one with respect to one subfraction that was present in less quantity in developing cells than it was in vegetative cells. This small difference may thus be due to the differing requirements of cells actively growing and cells not committed to a growth orientated cell cycle, further, from this evidence, it cannot be said with certainty that the basic nuclear proteins are involved in the nuclear control of deveopment. Analysis of the ribosomes of D.discoideum did however reveal stage-specific differences. Within the limits of the technique of competition hybridization, the rRNA moieties of ribosomes isolated from vegetative and developing cells were observed to be identical. The ribosomal proteins however did display a small, but reproducible degree of stage specificity. During development, ribosomes that were formed contained three ribosomal proteins that were not found to be associated with the ribosomes of vegetative cells. The developmental significance of these changes was established by the use of metabolically distinct cell populations that proceeded through the same developmental programme and were observed to exhibit the same changes in the ribosomal protein content between vegetative and developing cells. Further, the use of this approach has suggested that the presence of one other apparently stage-specific ribosomal protein is a function of the initial growth conditions of the cells and not of their subsequent differentiation. In conclusion, the advantages of a microbial system of development, such as the cellular slime mould D.discoideum, have enabled developmentally significant changes in the ribosomes to be observed which may be involved in the cytoplasmic control of the developmental programme. At the moment, the involvement of the basic nuclear proteins in the nuclear control of development is not firmly established, but it is hoped that in the future, this approach and this developmental system will provide answers to this and many other questions.