Isolation of a minichromosome containing the ribosomal genes from Physarum polycephalum

Abstract

The ribosomal genes of P. polycephalum are located on linear DNA molecules (rDNA) of 38 .times. 106 daltons present in each nucleoulus in 100-200 copies. The rDNA is not part of the large chromosomal DNA. These molecules have now been solubilized in the form of transcriptionally active deoxyribonucleoprotein complexes from nucleoli purified through Percoll gradients. This ribosomal chromatin (rDNP) sediments in sucrose gradients as homogeneous fraction with an approximate sedimentation coefficient of 100 S. It contains .alpha.-amanitin-resistant RNA polymerase I activity and rDNA as its major (or sole) DNA component. Buoyant density analysis indicates that ribosomal chromatin contains, besides RNA polymerase I, significant amounts of additional protein. RNA synthesized in vitro by the endogenous RNA polymerase is complementary to rDNA and is transcribed mainly from the sequences coding for 19S and 26S ribosomal RNA.