PubMed Health⌕ Search

Biomedical subjects

F L Yu

Publications and source records attributed to F L Yu.

50 records · Page 3Linked to original sources
PubMed1975

An improved method for the quantitative isolation of rat liver nuclear RNA polymerases.

Rat liver nuclear RNA polymerases exist in two functional states, one of which is active towards the endogenous chromatin template (engaged enzyme), while the other is inactive (free enzyme) (Yu, F.L. (1974) Nature 251, 344-346). This paper reports the direct separation of these two populations of RNA polymerases from isolated rat liver nuclei by a simple extraction procedure. It is estimated that as much as 50% of the total nuclear RNA polymerase activity in normal rat liver may exist in the form of the free enzyme. Evidence is also presented to indicate that the free enzyme activity is easily lost when the nuclear isolation procedure involves the use of an isotonic buffer medium, or when the isolated nuclei are subjected to sonication as is required for the solubilization of the nuclear RNA polymerases by the conventional method. Based on these new findings, it is proposed that nuclei be isolated directly in hypertonic sucrose and that the free enzyme be extracted before the nuclei are subjected to sonication to solubilize the engaged enzyme. This method circumvents the loss of the free RNA polymerase population and, as a result, the total yield of the nuclear RNA polymerases is greatly increased. The possible functional role of the free RNA polymerase in gene expression is discussed.

Animals↗
PubMed1972

The rapid turnover of RNA polymerase of rat liver nucleolus, and of its messenger RNA.

Turnover rates of the components of systems for RNA synthesis of rat-liver nucleus, nucleolus, and nucleoplasm were investigated. Cycloheximide administered in vivo selectively diminished nucleolar RNA synthesis in vitro. In contrast to the relatively stable nucleoplasmic RNA polymerase, nucleolar RNA polymerase (polymerase I) from rat liver decays rapidly upon cycloheximide administration, following pseudo-first order kinetics with a half-life of about 1.3 hr. Cycloheximide elicits this effect not through direct interaction with nucleolar RNA polymerase itself, nor by alteration of template function, but rather by inhibition of de novo synthesis of one or more of the protein components of nucleolar RNA polymerase. Similarly, when actinomycin-D was administered in vivo to inhibit RNA synthesis, the rate of decay of nucleolar RNA polymerase, assayed in the presence of exogenous poly d(A-T) template, was similar to that observed after cycloheximide administration. Thus, the messenger RNA(s) that codes for one or more of the catalytically essential polypeptide components of this enzyme turn over very rapidly with a half-life considerably shorter than 1.3 hr. The rapidity of turnover of both the enzyme protein and its messenger RNA(s) renders nucleolar RNA polymerase highly responsive to altered transcriptional, translational, or post-translational modulation. The marked differences in turnover rates of nucleolar and nucleoplasmic RNA polymerases indicate that at least certain of the protomeric components of nucleolar RNA polymerase I are distinct from those of nucleoplasmic RNA polymerases II and III.

Animals↗
PubMed1971

Cortisone stimulation of nucleolar RNA polymerase activity.

Four hours after intraperitoneal injection of cortisone acetate into rats, hepatic nuclei had an enhanced ability to incorporate labeled nucleoside triphosphates into RNA in vitro. When the nuclei were further fractionated into nucleolar and extranucleolar (nucleoplasmic) fractions, it was found that this hormonal effect was localized exclusively in the nucleolar fraction. This increased transcriptive activity potentially could be due to either (a) an increased availability of the DNA template or (b) an increase in the amount or catalytic efficiency of the RNA polymerase, or both. In order to distinguish between these possibilities, actinomycin D was used to inhibit the template function of endogenous nucleolar DNA and a synthetic template, polydeoxycytidylate, which is insensitive to actinomycin D and codes for polyriboguanylate synthesis, was used to evaluate RNA polymerase activity. These studies indicate that the increased RNA synthesis evident in nucleoli isolated from livers of rats four hours after cortisone treatment is largely a consequence of elevated levels of RNA polymerase.

Animals↗