The metabolism of heterogeneous nuclear RNA and the formation of cytoplasmic messenger RNA in animal cells

Messenger RNA regulation during compensatory renal growth

The normal pattern of mRNA metabolism almost certainly becomes altered after uninephrectomy, especially during the first day when ribosomes accumulate in the proximal tubular cells of the remaining kidney. For example, within the first hour after nephrectomy, the fraction of newly synthesized poly(A)-deficient mRNA increases relative to poly(A)-containing mRNA. Investigation of other growth-specific regulatory changes in renal mRNA has been complicated by its heterogeneity with respect to translational activity, polyadenylate content, membrane association, and cytoplasmic distribution. In general, analysis of kidney mRNA metabolism during growth has not been sufficiently thorough in that few timepoints have been examined after nephrectomy and the techniques used have been suited primarily to the study of only abundant mRNA sequences. Application of recombinant DNA methods should eliminate these difficulties and permit quantitative measurement of growth-specific genetic events during compensatory growth of the kidney.

Mechanism of linear DNA circularization: formation of 'lasso'-like structures of pre-mRNA DNA copies

An electron microscopic investigation of a DNA product synthesized on the pre-mRNA template using AMV reverse transcriptase in the absence of actinomycin D revealed linear single-stranded and double-stranded molecules, and, also, double-stranded branched as well as 'lasso'-like and circular structures. Models for the formation of such DNA structures are proposed.

Association of poly(adenylate)-deficient messenger ribonucleic acid with membranes in mouse kidney

To describe further the metabolism of messenger ribonucleic acid (mRNA) in mouse kidney, we examined newly synthesized mRNA deficient in poly(adenylate) [poly(A)]. Approximately 50% of renal polysomal mRNA that labeled selectively in the presence of the pyrimidine analogue 5-fluoroorotic acid lacks or is deficient in poly(A) as defined by its ability to bind to poly(A) affinity columns. Nearly one-half of this poly(A)-deficient mRNA is associated uniquely with a cellular membrane fraction detected by sedimentation of renal cytoplasm in sucrose density gradients containing EDTA and nonionic detergents. Poly(A+) mRNA and poly(A)-deficient mRNA [poly(A-) mRNA] have similar modal sedimentation coefficients (20-22 S) and similar cytoplasmic distribution. Although 95% of newly synthesized poly(A+) mRNA is released in 10 mM EDTA as 20-90 S ribonucleoproteins from polysomes greater than 80 S, only 55% of poly(A)-deficient mRNA is released under the same conditions. Poly(A)-deficient mRNA recovered from greater than 80 S ribonucleoproteins resistant to EDTA treatment lacks ribosomal RNA, is similar in size to poly(A+) mRNA, and is associated with membranous structures, since 70% of poly(A)-deficient mRNA in EDTA-resistant ribonucleoproteins is released into the 20-80 S region by solubilizing membranes with 1% Triton X-100. These membrane-associated renal poly(A-) mRNAs could have unique coding or regulatory functions.

Isolation and characterisation of a non-polyadenylated mRNA species with an affinity for poly(adenylic acid) from Friend leukaemia cells

Utilizing the technique of poly(A)-Sepharose affinity chromatography, it is possible to isolate a novel class of RNA molecules from polysomes of Friend leukaemia cells. These RNA species display messenger RNA-like behaviour. They are released from polysomes on treatment with EDTA and are able to direct polypeptide synthesis in a cell-free protein synthesising system. They appear to be distinct from the polyadenylated mRNAs, as judged by their lack of a 3'-terminal poly(A) tract, by their different size distribution, by their unusual base composition, by the presence of a possible 'uridylate rich' region towards their 3'-end, by their low sequence homology to polyadenylated mRNAs and by the difference in at least some of their translation products.

Stability of polyadenylic and polyadenylated ribonucleic acids in radish (Raphanus sativus) seedlings

The stability of polyadenylic acid and polyadenylated RNA was investigated in young radish (Raphanus sativus) seedlings. We first studied the decay of poly(A) content, using a [3H]poly(U) assay, following a complete block of transcription by cordycepin (200 microgram/ml). Two lifetime classes of polyadenylic acid have been determined in these seedlings: a short-lived component with a half-life of 30 min which represents 60% of poly(A) and a more stable component with varying half-lives of which the majority range from 4-10 h and a few are considerably longer. During this period rRNA was shown to decay linearly, taking about 41 h for half of this RNA to disappear. The life-time of the other moiety of polyadenylated-RNA was analysed by continuous labelling with [3H]uridine. We have been able to demonstrate that a significant part of the mRNA molecules turns over with a half-life similar to that of the more slowly turning-over poly(A). No evidence could be obtained for rapidly turning-over messenger RNA. Thus the rapidly turning over poly(A) could correspond to a poly(A) turn-over independent of the remainder of the sequence. When labelling was very long, an apparent steady-state was reached and we determined the polyadenylated RNA content of seedlings to be 2.2% of whole cell RNA. Finally, these results were compared with those previously obtained in studying early germination of radish embryo axes. In contrast with stored mRNA which is rapidly degraded following imbibition, part of the mRNA present in 22 h old seedlings is stable for several hours.

The effect of sodium chloride on the structure of ribonucleoprotein particles from rat liver nuclei

38S (monoparticles) and greater than 50--200S ribonucleoprotein particles (polyparticles) from rat liver nuclei were treated with increasing concentrations of sodium chloride. Treatment of 38S or greater than 50--200S particles, with 0.14, 0.25, 0.5, 1.0, and 2.0M NaCl resulted in a decrease of protein to RNA ratios from 8 to 3.1 for 38S particles and from 4.0 to 1.5 for greater than 20--200S particles. Correspondingly the densities in CsCl increased. Whereas the maximum of the sedimentation profile of polyparticles decreased from 90S to 50S after treatment with increasing NaCl concentrations, a discontinuous change was found in the case of monoparticles. It was shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis that the proteins which were dissociated by NaCl were in the molecular weight range of 30--45 000. Four of the 5 small molecular weight RNAs in the range of 4.5 to 8S remained tightly associated even after treatment of polyparticles with 2.0M NaCl. When 38S or 70--200S nRNP particles were exposed to increasing concentrations of NaCl (0.25, 0.5, 1.0, 2.0M), the molar ellipticity at 264 nm increased progressively to about 40%. Upon NaCl treatment of polyparticles and successive removal of the dissociated proteins by centrifugation the increase in the positive CD band at 264 nm was only 15%.

On pre-messenger RNA and transcriptions. A review

From the present review integrating old and new data emerge a few principles of gene expression in eukaryotes, and an infinite variety of possible mechanistic details generating the overal pattern. The few principles, most of which are not fundamentally new, may thus be summarized. 1) The eukaryotic genome is subdivided into transcriptional units: into transcriptons which are subject to individual activation controlled at DNA level. 2) Viral genomes may contain one or a few transcriptons, while cells of multicellular organisms contain from 3 x 10(3) in diptera up to an estimated 2 x 10(5) in birds and mammals. 3) Transcriptons may include one or several coding sequences. 4) Transcriptons vary considerably in size: in mammals and birds their size spectrum falls into the 2,000 to 20,000 bp range. 5) Units of coding information constituting one message (genes) and, possibly, units of regulative information are frequently broken up and stored within the transcripton in sub-genic blocks (of so far unknown significance) in general located at a certain distance from the 5' and 3' transcript terminals which are determined by the promotor and terminator signals. 6) The gene, in its specific definition as the functional unit underlying the phenotype, is in general constituted posttranscriptionally by the processing mechanisms from the mosaic of its genomic subunits in the transcripton; segments of coding, service and regulative sequences are recombined within themselves and with each other, polygenic transcripts separate into their unit messages. 7) Activated transcriptons produce pre-mRNA; these primary transcripts are colinear with the DNA of the transcriptional unit. 8) Primary pre-mRNA is processed into secondary pre-mRNA's by extragenic cleavage and intragenic ("splicing") processing, giving rise stepwise to functional mRNA. During this process chemical modifications as methylation, 5'-terminal capping and 3'-terminal polyadenylation take place. 9) Translation yields either potentially functional polypeptides or polycistronic polyproteins subject to further processing. 10) Processing is a regulated process; it involves many of the possible phases and mechanisms of post-transcriptional regulation (cf. 39, 40).

Message sequences are not adjacent to poly(A) in heterogeneous nuclear RNA of Friend leukemic cells

Hybridization of labeled RNA with excess amounts of unlabeled complementary DNA (cDNA) was used to investigate the location of cytoplasmic mRNA sequences in heterogenous nuclear RNA (hnRNA) of noninduced Friend leukemic cells. A heterologous hybrid between hnRNA and cytoplasmic cDNA was formed. Two homologous hybrids were also formed, one between poly(A)-containing mRNA and cytoplasmic cDNA, and the other between poly-(A)-containing hnRNA and nuclear cDNA. All hybrids were selected on hydroxyapatite columns after RNase treatment. The hybrids were further investigated for the presence of poly(A). No poly(A) was found in the heterologous hybrid, while both homologous hybrids contained poly(A). From these results we conclude that there exists a spacer nucleotide sequence between the poly(A) and the message sequences in hnRNA.

Studies on the preparation and properties of ribonucleoprotein particles from rat liver nuclei

Ribonucleoprotein particles of 38 S were extracted from rat liver nuclei with isotonic salt buffer under concomitant sonication. The fate of the endogeneous nuclear RNAases assayed with poly(A), high molecular weight yeast RNA and rapidly labeled hnRNA was followed during the preparation of 38-S nuclear ribonucleoprotein (nRNP) particles. Essentially all the RNAase activity could be removed from the particle preparation. The effect of synthetic RNAase inhibitors on the nRNP particles was studied. Upon extraction of nuclei with 0.14 M NaCl, approximately 38% of the total nuclear radioactivity was found in the 38-S nRNP particles. By two successive extractions of the remaining chromatin with either isotonic or 0.22 and 0.3 M NaCl, an additional 25 and 9% of rapidly labeled hnRNA of 38 S particle were dissociated from chromatin, respectively. The chromatin components, DNA, nonhistone proteins, histones and RNA were determined after successive salt extractions. Particularly alterations in the nonhistone proteins and RNA were found. The protein patterns upon SDS-acrylamide gel electrophoresis of the salt-extracted chromatin preparations were compared with those of the 38-S nRNP particles. Particularly proteins in the molecular weight range of 32 000-43 000 were dissociated from chromatin after treatment with 0.22 or 0.3 M NaCl.

Size distribution of rat liver nuclear RNA containing mRNA sequences

Total rat liver poly(A)-containing polysomal mRNA was size-fractionated on polyacrylamide gels in 98% formamide. Complementary DNA (cDNA) was prepared from the 8--14-S mRNA fraction and separated into sequences representing abundant and non-abundant mRNAs. The cDNA complementary to the abundant small mRNA of the rat liver cell (approximately 20 species) was hybridized to nuclear RNA of different lengths to determine the size distribution of nuclear RNA molecules which contain these messenger sequences. It was found that: 1. All abundant 8--14-S poly(A)-containing mRNAs have larger nuclear precursor molecules; 20% of the different messenger sequences are found in nuclear RNA of several times their cytoplasmic length. 2. 70% of the mass of the examined nuclear messenger sequences is in RNA molecules of a size similar to their polysomal mRNA; 30% are in larger than 18-S RNA and 2% are between 37 S and 44 S. 3. The majority of small messenger-containing RNA molecules in the RNA prepared from isolated nuclei are of true nuclear origin, since their frequency distribution differs significantly from that of the polysomal 8--14-S mRNA.