Ribosomal Subunit Exchange in Dictyostelium Purpureum

The rapid and frequent exchange of the 70S ribosomes in Escherichia coli, with their 50S and 30S subunits, has been established by kinetic studies and heavy isotope techniques (1-4) . While these studies were in progress, it was reported that ribosomes of Candida krusei, a eucaryote, undergo frequent exchange of ribosomal subunits (5) . It has been suggested that ribosomes dissociate and re-form between successive rounds of protein synthesis . The mechanism of polypeptide initiation in procaryotes has been well documented (6-8) . On the other hand, still very little is known about protein initiation in eucaryotic organisms . Early works on ribosomal synthesis in HeLa cells suggested that ribosomal subunits participate in polyribosome formation prior to being assembled in single monosomes (9, 10). It has also been shown that vaccinia-virus RNA becomes associated with the smaller ribosomal subunit before appearing on polysomes (11) . The cellular slime molds have been mainly studied as a model organism for problems concerning morphogenesis and differentiation (12, 13) . The life cycle can be easily subdivided into a phase of growth (cell division) and differentiation . During the growth phase, the amebas feed on bacteria and divide by mitosis approximately every 3 hr. These organisms thus offer great advantages to study ribosome dissociation and re-assembly by both kinetics and heavy isotope techniques .