Translational regulation of polysome formation during dormancy of Physarum polycephalum

The translational activity of actively growing microplasmodia and dormant microsclerotia of Physarum polycephalum was investigated by analyzing the distribution of ribosomes in polysomes. Microplasmodial post-mitochondrial fractions contained substantial amounts of polysomes and ribosomal subunits but very few native monosomes. During the starvation period which preceded microsclerotium formation, polysome levels remained constant, whereas the subunit titer began to increase. During encystment ribosomal subunits continued to accumulate as the level of polysomes gradually decreased. Dormant microsclerotia contained a large surplus of stored ribosomal subunits but no detectable polysomes. However, incubation of microsclerotia with concentrations of cycloheximide sufficient to slow polypeptide elongation without affecting initiation caused the gradual reappearance of polysomes at the expense of the subunits. Under these conditions the percentage of subunits driven into polysomes reached values similar to those of actively growing microplasmodia. Microsclerotia returned to nutrient medium contained very low levels of polysomes during the lag period which preceded germination. These were formed with preexisting, stored messenger ribonucleic acid. During the germination period, polysome levels were markedly increased. This elevation was dependent on new ribonucleic acid transcription. It is concluded that dormant microsclerotia contain functional messenger ribonucleic acid and ribosomes which are subject to translational repression at the level of initiation.