[CELL PHYSIOLOGICAL MECHANISMS IN THE REGULATION OF GENIC ACTIVITY IN THE GIANT CHROMOSOMES OF CHIRONOMUS THUMMI]

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Selective exposure of antigenic determinants in chromosomal proteins upon gene activation in polytene chromosomes

The immunological accessibility of the nucleosomal core protein H3 and non-histone protein, HMG-1, was studied in transcriptionally active regions of Chironomus thummi polytene chromosomes. Chromosomal loci were decondensed by incubating isolated salivary glands in various salt solutions or hemolymph. Indirect immunofluorescence studies on these chromosomes using anti-sera to histone H3 revealed that the puffed regions were depleted of fluorescence. The lack of fluorescence could be correlated with the degree of puffing and the level of transcriptional activity. The puffed regions fluoresce after anti-H3 addition if the chromosomes are not cross-linked with formaldehyde, and if prior to the addition of antibodies the chromosomes are treated with 45% acetic acid. We conclude that, whereas histone H3 is present in the puffed regions, its antigenic determinants are sterically hindered by components which are extractable by 45% acetic acid. On the other hand, the antigenic determinants of protein HMG-1 are always available to antibody binding in puffed regions, as well as other chromosomal areas.

Inhibition by tryptophan of nucleolar RNA synthesis in salivary gland nuclei of Chironomus thummi

Tryptophan, but not glutamine and lysine, inhibits the incorporation of 3H-uridine into nucleoli of explanted salivary glands by about 60%. In isolated nuclei also, the nucleolar incorporation of 3H-uridine triphosphate is reduced to about 50% by tryptophan. It is concluded that tryptophan acts directly at the nuclear level and the possible mechanisms.

The control of puffing by ions - the Kroeger hypothesis: a critical review

We have re-examined the several papers which appear to us to represent the principal lines of evidence for what we call the Kroeger hypothesis. To do this we have stated this hypothesis in its simplest, most concrete form, a form that has been repeatedly and forceably enunciated in the literature (Kroeger, 1963a, 1965, 1966, 1967, 1968; Kroeger and Lezzi 1966; Lezzi and Frigg, 1971). The evidence suggests to us that ecdysone's effect on puffing is probably not mediated by the [K+]/[Na+]. While such a model cannot, even now, be excluded, we see little reason to believe in it. We take the general issues raised by Kroeger's ideas very seriously. Nucleoprotein complexes are exquisitely sensitive to changes in salt concentration and ionic selectivity is a well-known property of proteins (and of ion-exchangers in general, see, for example, Diamond and Wright, 1969). Thus it might not be shocking if cells utilized this specificity in some general control over chromsosome structure, perhaps a second-layer of control superimposed upon other transcriptional controls. Therefore it is our feeling that Kroeger's data merits very careful and critical study, the more so because the experiments involved are intrinsically difficult. It is in this vein that we have tried to review Kroeger's data.

Puffing patterns during the fourth larval instar in Chironomus pallidivittatus salivary glands

The puffing pattern of polytene chromosomes in salivary glands from Chironomus pallidivittatus larvae and prepupae has been studied in glutaraldehyde-acetic acid fixed, lactic acid flattened preparations. Some observations were also made on F1 hybrid species C. pallidivittatus X C. tentans. Concerning the situation of puffing in Balbiani rings (BR), 2.783 chromosomes IV from 188 animals were scored. In standard 4th instar larvae, BR2 appears expanded, BR3 smaller but not collapsed and BRI either reduced of collapsed. During the first days following the red-head stage, which signals the beginning of the 4th instar, larvae show a large BR1; later it reduces and tends to collapse. At the end of the 4th instar, prepupae again may present an expanded BR1. On the contrary, the size of BR2 and BR3 remains unchanged from the red-head stage to the prepupa. A variable accumulation of droplets has been observed to occur in BR2 and BR1 from dated larvae and prepupae.--A characteristic pattern of puffing was found in prepupae, which consisted in the appearance of conspicuous puffs at regions I-6B, I-7B, I-7B, I-18C, III-9B and IV-4B. Puffs at I-2B, I-3B, I-9A,I-11C,II-4A, and IV-4B were observed during most of the 4th larval instar, as well as in late larvae and prepupae. Among all these puffs, those at I-7B, I-9A, I-17B, and IV-4B frequently showed variable amounts of droplets.

Environmental influences on puffing in the salivary gland chromosomes of Drosophila melanogaster

The sequence of development of salivary gland chromosome puffs inDrosophila melanogastercultured on live yeast and RNA-deficient medium has been studied in late larval and prepupal stages. Differences in puffing pattern between individuals cultured in the two environments are almost entirely due to shifts in the relative timing of puff development at specific chromosome sites. Detailed studies on the right arm of the second chromosome indicate that, at certain sites, puffing activity spreads to adjacent lettered subdivisions of the chromosome. There are differences in the extent of lateral spreading of activity into adjacent regions in the two environments which may be due to clustering of functionally related genes.

Control of gene activities in the polytene chromosomes of Chironomus tentans by ecdysone and juvenile hormone

The injection of ecdysone (molting hormone) into chironomid larvae is known to result in the stimulation of specific gene activities (increased RNA synthesis). The morphological manifestation of this gene activation is the formation of a puff. Cytological and autoradiographic analysis of several regions of the salivary gland chromosomes of Chironomus tentans revealed that ecdysone also stimulates Balbiani ring 1. In contradistinction to the puffs, the Balbiani rings are tissue specific and account for a significant percentage of the cell's non-nucleolar RNA.The application of juvenile hormone results in a decreased activity of Balbiani ring 1, suggesting that the two hormones may act antagonistically. In addition, juvenile hormone induces a puff at chromosome region I-19-A and this puff becomes less active after a short-term treatment with ecdysone. These data demonstrate an effect of juvenile hormone on puffing and are the first to demonstrate activation of a Balbiani ring by ecdysone.

Observations on the nucleolar and total cell body nucleic acid of injured nerve cells

1. The nucleic acid content of neuronal nucleoli and the total cell body nucleic acid content of neurones of the hypoglossal nucleus were measured by ultraviolet absorption microspectrography.2. After nerve injury both the nucleolar nucleic acid and the total cell body nucleic acid increased: nucleolar changes preceded those of the cell body.3. The closer to the nerve cell body that the axon was injured the earlier was the onset and the decline of the nucleolar response.4. Actinomycin D was given to prevent DNA-primed RNA synthesis, and the rate of ;decay' of nucleolar RNA was measured. This rate varied after nerve injury and was closely related to the nucleolar nucleic acid content.5. The apparent rate of transfer of labelled RNA from the neuronal nucleus into the cytoplasm changed after nerve injury in a manner closely related to the changes in nucleolar nucleic acid content.6. It was demonstrated by making consecutive nerve injuries or by preventing or delaying nerve regeneration, that the nucleic acid changes were not induced by removal of contact between the neurone and its motor end-plate, and were not repressed by the restoration of such contact.7. When regeneration was prevented the nucleolar nucleic acid content and the total cell body nucleic acid ultimately decreased to values less than normal: this decrease was greater when more of the axon was initially removed.8. The results are discussed in relation to the factor responsible for derepression and repression of DNA cistrons for ribosome synthesis in injured nerve cells.

Permeability of a nuclear membrane: changes during normal development and changes induced by growth hormone

The ion permeability of the nuclear membrane envelope of salivary gland cells (of the midge Chironomus thummi) undergoes changes during development. Following the early fourth instar stage, permeability falls to about one-fifth of its base value over a period of 3 to 5 days of development of the animal, and then rises again over the next 2 to 4 days. The falling phase of the change can be reproduced within 1 hour by injections of the growth hormone ecdyson.