Characterization of thermosensitive mutants of Physarum polycephalum

Abstract

Amoebal thermosensitive mutants of Physarum polycephalum have been isolated after mutagenesis of the amoebal form by nitrosoguanidine treatment. About 70% of the independent thermosensitive amoebal mutants obtained were also thermosensitive in the plasmodial form. Two basic screening methods were applied at the same time to thermosensitive microplasmodia in order to detect strains defective in premitotic events, mitosis or chromosomal DNA synthesis. The first method consists in the determination of increase in protein. RNA and DNA with incubation time at the non-permissive temperature. It allowed the detection of four independent thermosensitive mutant strains, showing an early arrest in DNA synthesis. The second one is the quantification of the variations of the different nuclear types at the restrictive temperature. Two mutant strains presented very large nuclei, uni- or multinucleolate, very similar to those obtained after methyl benzimidazole carbamate treatment, suggesting a defect in one of the mitotic processes. One of these two mutant strains showed an early arrest in DNA synthesis at the restrictive temperature. These two screening procedures were completed by electron microscopic observation. This technique allowed the detection of intra-nuclear macrotubular crystal-like structures in a thermosensitive mutant showing a reduced DNA synthesis at the non-permissive temperature.