THE CELLULAR SITES OF SYNTHESIS OF RIBOSOMAL AND 4S RNA

Abstract

In previous publications, -4 we presented evidence which indicates that major portion of cytoplasmic RNA (C-RNA) is derived from RNA which is synthesized in nucleolar region of nucleus (n-RNA). Furthermore, it was proposed that n-RNA and RNA of extranucleolar parts of nucleus (N-RNA) are synthesized independently of each other. These studies, carried out on level of individual cell, did not distinguish between different molecular species of RNA such as ribosomal, amino acid-accepting (transfer), and messenger RNA. A general relationship, such as C-RNA is mostly ribosomal RNA, can readily be inferred but there is as yet no description of RNA synthesis which adequately comprehends both cytological and molecular terms. Some progress along these lines has been made by Georgiev and collaborators. ' These investigators used different phenol extraction procedures to separate partially various classes of RNA and deduced from an analysis of incorporation kinetics that an RNA, from what they term the chromosomal-nucleolar apparatus, not extractable by phenol at pH 6.0-0.14 M NaCl, is a precursor of high polymeric cytoplasmic RNA. Other nuclear fractions were described, one of which is low polymeric and capable of an in vivo incorporation of p32 which is 20 times faster than a similar low polymeric cytoplasmic fraction. Unfortunately, no clear distinction is made between nucleolar and chromosomal RNA.