The kinetics of labelling and intracellular distribution of mammalian DNA-like RNA isolated by methylated albumin kieselguhr column chromatography

Abstract

1. 1. Methylated albumin kieselguhr column chromatography has been used to follow the kinetics of incorporation of labelled precursors into total and cytoplasmic DNA-like RNA. 2. 2. After a 5 min labelling period 81 % of the radioactivity is incorporated into two main species of DNA-like RNA. 43 % of this label is present as the larger DNA-like RNA which elutes towards the end of a NaCl concentration gradient (previously termed Q2), and the remaining 38 % of the label is present as DNA-like RNA which is tenaciously bound to the column after elution with salt (termed TD RNA). 3. 3. The results of longer labelling periods show that both species of DNA-like RNA are turning over more rapidly than rRNA and that Q2 is turning over more rapidly than TD RNA. 4. 4. No Q2 RNA was found in the cytoplasm of either liver or cells in culture. The percentage of TD RNA in labelled cytoplasmic RNA is less than the percentage of TD RNA in labelled total RNA. These results illustrate the danger of equating rapidly labelled DNA-like RNA with mRNA. 5. 5. A small amount of another DNA-like RNA species was also found in the cytoplasm. This DNA-like RNA has yet to be characterised, but it originates from either Q2 or TD RNA. 6. 6. Low concentrations of actinomycin D were used to follow the kinetics of labelling of the DNA-like RNA species without interference from labelled rRNA. The results of chasing labelled DNA-like RNA (taken with similarities of base composition and structure) indicate that Q2 is a precursor of at least part of TD RNA, which is then eventually broken down to acid-soluble products. 7. 7. These results are in agreement with some of the current theories on the role of DNA-like RNA in the regulation of gene expression.