Heat shock-induced appearance of RNA polymerase II in karyoskeletal protein-enriched (nuclear "matrix") fractions correlates with transcriptional shutdown in Drosophila melanogaster.

Heat shock in vivo or brief incubation at moderately elevated temperatures (15 min at 37 degrees C) in vitro is required for the structural stability of proteinaceous karyoskeletal elements purified from Drosophila melanogaster (McConnell, M., Whalen, A. M., Smith, D. E., and Fisher, P. A. (1987) J. Cell Biol. 105, 1087-1098). We have found that the appearance of the two largest subunits of RNA polymerase II in karyoskeletal preparations is promoted by conditions identical to those which promote in vitro stability of karyoskeletal elements overall. Increased levels of polymerase II in karyoskeletal protein-enriched fractions correlate with decreased levels of nucleotide incorporation in nuclear transcription run-on assays. These results are not easily reconciled with the proposal that putative karyoskeletal elements such as the internal nuclear "matrix" are sites of active transcription in vivo.