NUCLEOLUS-LIKE MORPHOLOGY PRODUCED DURING THE IN-VITRO REASSOCIATION OF NUCLEOLAR COMPONENTS

Nucleoli, the sites of rRNA synthesis, rRNA processing, and the assemb ly of ribosomes, are dynamic organelles that, in most cells, disperse and reform during mitosis. The mechanisms that regulate nucleolar form ation are unknown as is the relationship between nucleolar morphology and the pathway of ribosome biogenesis. In this report we describe the in vitro formation of nucleolus-like particles (NLPs) from soluble ex tracts of nucleoli. NLPs, which reached sizes comparable to nucleoli ( 1-3 mum), were found to contain 40% of the nucleolar DNA, RNA, and pro tein. The ultrastructure of NLPs resembled that of a number of in vivo structures including compact nucleoli, prenucleolar bodies, and pseud onucleoli. The particles were composed of two morphologically distinct regions. The core resembled the dense fibrillar component (DFC) of nu cleoli while the cortex resembled the granular component (GC) of nucle oli. The cortex of NLPs contained numerous 15-20 nm osmophilic granule s that resembled the preribosomes found in the GC of nucleoli. The dis tribution of nucleolar proteins in NLPs also resembled that in nucleol i. BN46/51, a component of the GC of nucleoli, was restricted to the G C-like cortex of NLPs. A mAb that bound to the DFC of nucleoli, bound only to the DFC-like core of NLPs while a second mAb that bound to bot h the DFC and GC of nucleoli, bound to both the core and cortex of NLP s. Thus solubilized components of nucleoli can reassociate in vitro to produce particles that resemble nucleoli in their size, ultrastructur e, and protein distribution.