Isolation of an MSP1-derived transcriptionally active nucleolar particle.

A discrete macromolecular structure with inherent transcription activity has been isolated from Novikoff nucleoli. The particle (350 A diameter) was released from the intact nucleoli by digestion with restriction endonuclease (Msp1) and is enriched in repetitive DNA sequences both 5' and 3' to the ribosomal genes. In vitro transcription analyses suggest that the particle contains the rDNA polymerase template, and that its 12-15 major proteins are responsible for transcriptional activity without added soluble protein factors.