STUDIES ON ISOLATED NUCLEI

Abstract

This paper describes the subfractionation of nuclei isolated from guinea pig liver by the procedure presented in the first article of the series (8). Centrifugation in a density gradient system of nuclear fractions disrupted by sonication permits the isolation of the following subtractions (a) a nucleolar subtraction which consists mainly of nucleoli surrounded by a variable amount of nucleolus-associated chromatin and contaminated by chromatin blocks derived primarily from von Kupffer cell nuclei; (b) and (c), two nucleoplasmic subtractions (I and n) which consist mainly of chromatin threads in a coarser (I) or finer (n) degree of fragmentation. The protein, RNA, and DNA content of these subtractions was determined, and their RNA''s characterized in terms of NaCl-solubility, nucleotide composition, and in vivo nucleotide turnover, using inorganic 32P as a marker. The results indicate that there are at least three types of RNA in the nucleus (one in the nucleolus and two in the nucleoplasm or chromatin), which differ from one another in NaCl-solubility, nucleotide composition, turnover, and possibly sequence. Possible relations among these RNA''s and those of the cytoplasm are discussed.