Thermal diminution and augmentation of the retention of transportable rRNA in nuclear envelope-free nuclei.

We have examined the effect of temperature on the rRNA transport from nuclei isolated from Tetrahymena after removal of both nuclear membranes and pore complexes by 1% Triton X-100. These nuclei export rRNA as precursor ribosomal ribonucleoprotein particles at both 28 degrees C and 8 degrees C which are qualitatively the same in terms of rRNA pattern, sedimentation coefficients and buoyant densities. At 8 degrees C, however, significantly fewer ribosomal ribonucleoprotein particles can be maximally exported than at 28 degrees C, though nuclei contain enough potentially transportable particles. These are increasingly released with increasing temperatures. Under conditions non-permissive for export, temperature elevation decreases the number of the potentially transportable ribosomal ribonucleoprotein particles in nuclei. Our data show: transportable ribosomal ribonucleoprotein particles inside nuclei are not 'free', but rather are subject to a complex temperature-sensitive retention: this retention is gradually diminished under export conditions and augmented under non-permissive export conditions with increasing temperatures. These retention mechanisms operate at an intranuclear level preceding the ribosomal ribonucleoprotein passage through the nuclear envelope pore complexes, i.e., the nuclear envelope regulates neither the number of potentially transportable ribosomal ribonucleoprotein particles in nuclei nor the number of those particles which can be maximally exported from nuclei at a given temperature. We suggest that these retention mechanisms involve temperature-sensitive domains of the nuclear matrix.