Kinetic evidence that a discrete messenger-like RNA is formed by post-transcriptional size reduction of heterogeneous nuclear RNA

Two transcripts of an individual Balbiani ring from the salivary gland cells of Acricotopus lucidus (Diptera, Chironomidae)

The polytene chromosomes in main lobe cells of the Acricotopus salivary gland carry two large Balbiani rings (BR1, BR2), which BR1 exhibits the higher incorporation rate of tritiated nucleosides. The size of BR1 varies in conjunction with a subterminal inversion which includes the BR1 site. BR1 in both the homozygous and heterozygous inverted arrangement (BR1A and BR1B, respectively) is consistently smaller than in the homozygous standard arrangement (BR1C). The RNA content of BR1A and BR1C (3.1 and 6.7 pg, respectively) corresponds well with their relative size.--Using the method of micromanipulation of squashed salivary gland chromosomes the RNA transcripts of BR1A and BR1C have been extracted from microdissected Balbiani rings and fractionated by electrophoresis on 1% agarose gels. In BR1A one RNA fraction with a molecular weight of about 40 x 10(6) D is transcribed. The gel electrophoretic pattern of BR1C is characterized by an additional RNA fraction of 5 x 10(6) D. The main part of BR1 RNA is bound to oligo(dT)-cellulose suggesting the adenylation at the chromosome of Balbiani ring RNA.--In the profile of poly(A)-containing cytoplasmic RNA obtained after in vivo labelling two peaks can be detected which correspond in size to the BR1 RNA. These putative BR1 transcripts accumulate to a high concentration in the cytoplasm and show a relatively low stability during in vivo chase experiments.

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).

The size distribution of poly(A) in newly synthesized and old Balbiani ring RNA

The size distribution of newly synthesized and old poly(A) sequences on transcripts of the giant tissue specific puffs, Balbiani rings in salivary glands of Chironomus tentans has been determined. After labeling with [3H]adenosine, poly(A) containing Balbiani ring RNA(75S RNA) was selectively collected by means of a recently developed technique. This combines electrophoretic fractionation and affinity chromatography in one run by insertion of poly(U) immobilized in glass fiber filters in an agarose gel slab. The majority of short-term labeled poly(A) chains released from poly(A) containing 75S RNA molecules is distributed within a narrow size range migrating as one peak with a mean value of 103 +/- 2 nucleotides, which is probably the initial length of poly(A). The labeling pattern of ribonuclease resistant poly(A) stretches after chase with unlabeled adenosine displays a relatively broad and heterogeneous size spectrum from at least 20 to more than 100 nucleotides. The main peak of labeled adenylate core in newly formed poly(A) containing RNA of non-Balbiani ring origin is dispersed within a broader size range than that of Balbiani ring RNA and possesses an average value of 94 +/- 2 nucleotides. During chase conditions, the relative frequency of occurrence of poly(A) chains of 75S RNA in the size range of 100 nucleotides exhibits a significant decrease in parallel with a rather uniform gain in the size classes between 20--50 nucleotides. However, the results are inconsistent with the existence of an age-dependent shortening of poly(A) chains in the balbiani ring RNA. A significant portion of 75S RNA molecules remain associated with poly(A) segments which are essentially of original size even after 21 hr in the presence of unlabeled adenosine. This finding provides support for the possibility that the initiation of the poly(A) shortening in 75S RNA is a stochastic process.