This review concern recent experimental information areas of animal cell molecular biology which are relevant to the mechanism of gene regulation. New data regarding interspersion and clustering of repetitive sequence elements in DNA are considered Molecular characteristics of
animal structural genes and mRNAs are discussed, with particular reference the frequency of structural gene sequences, mRNA turn over and the interpretation of dipteran complementation groups. The molecular characteristics of nuclear RNAs, the primary transcription products, are reviewed. Evidence for transcription level regulation is summarized and the relation of nuclear
and mRNA examined. The protein activator branch of the Britten-Davidson model for gene regulation is further developed and considered in light of current knowledge.

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Organization, transcription, and regulation in the animal genome.

The experimental description of the mechanics as well as the regulation of bacterial protein synthesis appears to have entered a final stage. Virtually every step involved in the assembly of amino acids into protein-chain initiation, elongation, termination-can be performed in the test tube wi-th components purified from bacterial cells (1). The steps in bacterial messenger RNA (mRNA) biogenesis are almost as well understood. The functional RNA polymerase and several regulatory proteins have been isolated and shown to participate in the control of synthesis of specific mRNA, both under conditions where only RNA is produced and in coupled systems where all the steps occur between the initiation of mRNA production and completion of functional enzyme protein (1). These awesome biochemical achievements with bacterial preparations probably would not have been possible, certainly not at this time, if a decade of successful bacterial genetics had not provided evidence concerning the mode of regulation that exists-namely, control of transcription of mRNA by regulatory proteins. Presented with the challenge of understanding protein synithesis in mammalian cells in detail comparable to the I-ent understanding in bacteria, the ':eh biologist might, after a first appraisal, despair. The genetic methods for isolating and characterizing mutants which might affect the availability of mRNA do not exist for mammalian cells and, if developed, will certainly present greater practical difficulties in the-characterization and experimental

https://science.sciencemag.org/content/sci/181/4106/1215.full.pdf

Biogenesis of mRNA: Genetic Regulation in Mammalian Cells

An average size chromomere of the polytene X chromosome of Drosophila melanogaster contains enough DNA in each haploid equivalent strand to code for 30 genes, each 1,000 nucleotides long. We have attempted to learn about the organization of chromosomes by asking how many functional units can be localized within a chromomere. This was done by 1) recovery of mutants representative of every cistron in the 3A2-3C2 region; 2) the characterization of the function of each mutant type and grouping by complementation tests; 3) the determination of the genetic and cytological position of each cistron by recombination and deletion mapping. The data clearly show one functional group per chromomere. It is postulated that a chromomere is one cistron within which much of the DNA is regulatory in function.

https://www.genetics.org/content/genetics/71/1/139.full.pdf

The anatomy and function of a segment of the X chromosome of Drosophila melanogaster.

Biochemical and genetic studies have produced a vast fund of knowledge concerning gene action and regulation in prokaryotes In these organisms the DNA is exposed rather nakedly to the world, protected primarily by the cell membrane In eukaryotes the DNA seems far better shielded, being enmeshed in histone and nonhistone proteins and sequestered behind both the cell and the nuclear membrane These differences have led to a considerable degree of caution in the application of this knowledge of prokaryotes to problems of gene regulation in eukaryotes, and rightly so There are, however, several observations which suggest that higher organisms may have picked up a number of fundamental genetic tricks from their lowly predecessors It has frequently been suggested that eukaryotes must do things differently from prokaryotes, until proven otherwise It may be prudent to reverse this line of thought and suggest that they do things the came until proven different The following similarities suggest this (1) T he basic genetic dogmas concerning DNA replication, transcription, andlation are similar, (2) The genetic code is the same (3) Both systems appear to make use of cyclic AMP as a basic mediator for, humoral or diffusible, signals (4) In both systems DNA synthesis may be controlled at membranes (5) Both make use of different types of RNA polymerase and RNA polymerase cofactors (6) Recent studies of polylysine binding to chromatin suggest the eukaryotic DNA may not be so thoroughly enmeshed in Protein as once thought (7) The visualization of genes in action by electron microscopic techniques intimates that genes are spaced and read in a similar manner And finally, (8) merely because the clustering of related genes is unusual in higher organisms is no reason in itself to totally discard the promoter-operator-repressor concept as a way of regulating single structural genes This system has provided an immense amount of data concerning the manner in which proteins interact with specific DNA sequences to control the attachment and utilization of RNA polymerase It is hard to imagine that eukaryotes, being presented with such a superb mechanism for controlling DNA transcription, would totally discard it and opt for something different It is far more likely that they would build on to this solid foundation

The structure and function of chromatin.

Sequential gene activation by ecdysone in polytene chromosomes of Drosophila melanogaster: II. The effects of inhibitors of protein synthesis

Electrophoretic characterization of nucleolar RNA from Chironomus tentans salivary gland cells.

The presence of heterogeneous RNA of high molecular weight has been demonstrated on the giant chromosomes, in the nuclear sap and in the cytoplasm of the salivary glands in Chironomus tentans. The kinetic properties of this heterogeneous RNA have also been outlined in some detail. — Salivary glands were incubated for different time intervals (20, 45 and 180 min) in haemolymph, supplemented with tritiated cytidine and uridine. The different cellular components were isolated by micromanipulation and RNA extracted with an SDS-pronase solution and analysed with electrophoresis in agarose. — Heterogeneous, high molecular weight RNA with a peak around 35 S was saturated with label on chromosome I, II and III in 45 min, although the synthetic capacity was unchanged during at least 180 min incubation. This indicated a complete turnover of heterogeneous RNA on the chromosomes in less than 45 min. The turnover time in the giant puffs (the so called Balbiani rings) on the fourth chromosome, was even shorter and estimated to less than 30 min. No shift in the electrophoretic pattern of this heterogeneous RNA was found to occur on the chromosomes during long incubation times or during actinomycin D experiments. These labelling characteristics of heterogeneous RNA on the chromosomes indicate that all the different molecules in the heterogeneous RNA have a similar and rapid turnover. A conversion to smaller, stable molecules was excluded. — Heterogeneous RNA of a distribution corresponding to that on the chromosomes was found in the nuclear sap and also in the cytoplasm. The activity in both these cellular compartments increased between 45 and 180 min incubation. The distribution pattern for high molecular weight RNA was in all experiments similar on the chromosomes, in the nuclear sap and in the cytoplasm. It appears that at least a considerable part of the high molecular weight RNA leaves the chromosomes to enter the nuclear sap and lateron to some extent the cytoplasm in this high molecular form. Stable molecules of smaller size (6–15 S) did not appear during 180 min incubation. The data indicate, however, also a substantial breakdown of heterogeneous RNA to acid soluble products during this time.

Chromosomal RNA synthesis in polytene chromosomes of Chironomus tentans.

Rapidly labelled RNA in Balbiani ring 2 on chromosome IV in the salivary glands of Chironomus tentans was investigated. This RNA is likely to be transcribed from only one chromosomal band, supposed to be a single operational unit in these polytenic cells (Beermann, 1966).

U. Ringborg

Papers

Electrophoretic characterization of nucleolar RNA from Chironomus tentans salivary gland cells.

Chromosomal RNA synthesis in polytene chromosomes of Chironomus tentans.

RNA synthesis in a Balbiani ring in Chironomus tentans salivary gland cells.

Evidence for transport of preribosomal RNA from the nucleolus to the chromosomes in Chironomus tentans salivary gland cells.

Low molecular weight RNA in cell components of Chironomus tentans salivary glands.