In vitro nuclear transport of ribosomal ribonucleoprotein: temperature affects quantity but not quality of exported particles.

The in vitro export of ribosomal ribonucleoprotein (rRNP) from Tetrahymena nuclei was investigated at the optimal growth temperature of 28 degrees C and at the nonlethal temperature of 8 degrees C. At both temperatures, nuclei exported ribosomal precursor particles that revealed the same physical qualities of size, appearance in negative-staining electron microscopy, sedimentation coefficient, buoyant density, and rRNA pattern. Surprisingly, fewer rRNP particles were exported at 8 than at 28 degrees C, as was revealed by a lower saturation plateau in the export kinetics from nuclei prelabeled with [3H]uridine. Upon a temperature increase from 8 to 28 degrees C, additional rRNP particles were exported. We conclude that nuclei export only a defined portion of rRNP particles at a given temperature, although enough potentially transportable rRNP particles are present in nuclei. Obviously, the reactivity of at least one of the reactants involved directly or indirectly in rRNP export changes with temperature.