The effects of heat shock on development and protein synthesis of the myxomycete, Physarum polycephalum : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University

Abstract

When Drosophila embryos and pupae, undergoing differentiation, are subjected to a brief heat shock at an elevated temperature, specific abnormalities are produced in the adult organism. The type of abnormality produced is dependant upon the stage in the differentiation process at which the heat shock is administered. In addition, Drosophila cells respond to heat shock with the rapid cessation of all pre-existing transcription and translation, and the simultaneous transcription of a specific set of heat shock genes. Heat shock mRNAs are subsequently translated preferentially into a novel set of heat shock proteins. This response of Drosophila is independant of the type of tissue and the developmental state. Many other organisms display a similar protein synthetic heat shock response. Physarum plasmodia can be readily induced experimentally to undergo a process of differentiation leading to the development of mature sporangia. Heat shocks administered during this differentiation have been reported to result in the formation of abnormal sporangia. This thesis is concerned with the detailed investigation of normal sporangial development in Physarum and the effects on subsequent development of heat shocks administered at a number of different stages of development, using a combination of light microscopy, scanning electron microscopy and transmission electron microscopy. The more immediate response of Physarum protein synthesis to heat shock is also investigated, using radioactive labeling of proteins, SDS-polyacylamide gel electrophoresis and fluorography. Heat shocks administered during the early stages of Physarum sporangial development induce a delay in subsequent development but normal mature sporangia are produced. Heat shocks administered late in development induce the formation of grossly abnormal sporangia, with the type of abnormality induced being dependant upon the stage of development attained at the time of the heat shock. Heat shocks administered at a mid-point in development induce a complete, though not permanent, developmental arrest. Heat shocks at a number of different stages of Physarum sporangial development induce a considerable reduction in pre-existing protein synthesis, while the synthesis of a novel set of heat shock proteins is induced by each heat shock. The heat shock proteins of Physarum have approximate molecular weights of 85,000, 78,000, 75,000, 73,000, 69,000, 18,000, and 14,000 daltons, with the predominant heat shock protein being that of 69,000 daltons. Physarum plasmodia undergoing active growth synthesize the set of heat shock proteins typical of plasmodia undergoing development but, in this case, the pre-existing protein synthesis continues during the heat shock. The effects of heat shock on both the development and protein synthesis of Physarum plasmodia are discussed and relationships between these two phenomena are proposed. Also discussed are the similarities between heat shock-induced abnormalities in Drosophila and Physarum, and the similarities between the protein synthetic heat shock responses of these two organisms. In both organisms, a similar molecular basis probably underlies abnormality production, while the major heat shock proteins of both organisms are remarkably similar in molecular weight.