Thermal down‐regulation of exportable rRNA in nuclei

The export of rRNP particles from nuclei isolated from Tetrahymena was investigated after preincubating the nuclei at different temperatures under nonpermissive export‐conditions. We observed a new phenomenon: Temperature elevation from the sublethal cells' growth temperature, 8°C, to the optimal temperature, 28°C, lead to a gradual down‐regulation in the maximal proportion of rRNP particles subsequently exported from nuclei at 28°C. This thermal down‐regulation is apparently not due to qualitative changes in the exported rRNP particles, a derangement in the gross nuclear organization, a degradation and/or nicking of the nuclear rRNA, a gross decomposition of the major nuclear proteins, a random cross‐linking of nuclear components by disulfide bonds, or an elution of nuclear factors possibly required for rRNP export. Moreover, there is a corresponding thermal down‐regulation in nuclear envelope‐free nuclei. Our data indicate that nuclei possess a mechanism that regulates the number of potentially exportable rRNP particles at a level preceding the rRNP passage through the nuclear envelope.