The control of puffing in Drosophila hydei

Replication of DNA in larval salivary glands of Drosophila after in vivo synchronization

The replication of DNA in the giant chromosomes in different cells of Drosophila larval salivary glands is asynchronous. A method of in vivo synchronization of the nuclei has been successfully devised by a 5'-fluorodeoxyuridine (FdU) block-release-thymidine chase technique, and the patterns of replication sequences have been examined by 3H-thymidine autoradiography. When the larvae of Drosophila melanogaster are fed on FdU for 48 h, and the block is released thereafter, most cells are found in mid-replication phase (termed 3C). When the larvae are subjected to a chase in normal Drosophila medium (or sucrose), a series of cells arrive at 3C phase about every 8 h. When they are chased in sucrose containing thymidine, the number of cells in 3C phase rises to 70%, and then drops rapidly to 1-2% of all labelled cells. The terminal phases (3D, 2D and 1D) reach a peak between 4-8 h. At 12-14 h of chase the 3D-1D peaks decline and a third peak consisting mostly of the initial phases (DD-1C) is found at 14--16 h. The replication of DNA in polytene chromosomes of Drosophila thus seems to proceed in a regular sequence of DD-3C-1D.

Identification of the Balbiani ring 2 chromomere and determination of the content and compaction of its DNA

The giänt puff Balbiani ring 2 (BR 2) in the salivary glands of Chironomus tentants is known to contain transcriptionally active 75S RNA genes. The corresponding chromosomal superstructure, the BR 2 chromomere, has been identified and characterized in the present study. It was demonstrated by cytological methods that BR 2 originates from the broad band IV-3B10, since all the bands except the 3B10 band could be detected as intact bands in the vicinity of BR 2. In situ hybridization of BR 2 RNA to squashed Malpighian tubule chromosomes lacking BR 2, showed that all or almost all the DNA sequences complementary to BR 2 RNA are located in the 3B10 band. The DNA amount in the 3B10 band, designated the Br 2 band, was measured in relation to the DNA content of the whole chromosome set by both direct and indirect microspectrophotometry of Feulgen-stained Malpighian tubule chromosomes. From a determination of the haploid DNA content in sperm cells (0.25 pg), the amount of DNA in at BR 2 chromomere could then be calculated to be 5.1 x 10-4 pg DNA or 470 kb DNA. A minimum value of the DNA compaction within the BR 2 chromomere was estimated to 380 from the B-form length of BR 2 DNA (160 mum) and the thickness of the BR 2 band (0.42 mum). Since there are only between 1 and 4 75S RNA genes, 37 kb in size, in a BR 2 chromomere, most of the BR 2 DNA must consist of DNA not coding for 75S RNA. The nature of this DNA is discussed in relation to the high average AT-content of Chironomus DNA. The orgnization of the chromosome fiber in the BR 2 chromomere is considered in relation to the gene activation process in the salivary glands, i.e. the formation of a 75S RNA transcription loop from a protion of the tightly packed chromosome fiber in the BR 2 chromomere.

RNA polymerase B (or II) in heat induced puffs of Drosophila polytene chromosomes

Using indirect immunofluorescence visualization techniques we investigated the distribution of RNA polymerase B (or II) and histone H1 at heat shock puff loci in Drosophila melanogaster polytene chromosomes at different times during and after heat shock. After heat treatments of from 5 to 45 min, the heat shock puff displayed intense fluorescence when stained for RNA polymerase B, but relatively little fluorescence when stained for histone H1. Returning heat shocked larvae to room temperature resulted in the appearance of a distinctive pattern of RNA polymerase-associated fluorescence in the heat shock puff at 87C, presumably reflecting events associated with the inactivation and regression of this puff. Large differences observed in the apparent RNA polymerase B content of puffs of similar size suggest that the interaction of RNA polymerase B with chromosomal loci does not depend on simply the state of condensation or decondensation of the chromatin.

Gene control in eukaryotes and the c-value paradox "excess" DNA as an impediment to transcription of coding sequences

Ways in which control of gene activity may lead to the observed high DNA content per haploid eukaryote genome are examined. It is proposed that deoxyribonucleoprotein (DNP) acts as a barrier to transcription at two distinct structural levels. At the lower level, melting of the nucleosome supercoil (quaternary structure) and of the nucleosomes (tertiary structure) might be brought about by the process of transcription itself. After unwinding the barrier section, the polymerase would eventually reach the structural gene. The transcripts of noncoding sequences, at least as far as their "unique" sequence components are concerned, may thus have filled their main function through the very process of transcription. The possibility of an inverse relationship between the length of the DNP barrier and the rates of transcription of the coding sequences is to some extent supported by available data. Different modes of coordination between the transcription of mRNA and of hnRNA from a single functional unit of gene action (funga) are considered. An analysis of gene control at high structural levels of DNP is made on the basis of other data, in relation to the concepts of eurygenic and stenogenic control. The concept of a euryon is introduced, namely of a set of linked fugas under common eurygenic control. Structure of order higher than quaternary can be inferred to exist in larger chromomeres of polytene chromosomes and in corresponding sections of ordinary chromosomes. Only moderate amounts of highest order interphase euchromatic structure are likely to be able to be accomodated in average chromomeres and none in very thin chromomeres. Puffs are interpreted as the melting of highest order interphase structure, and the absence of puffs during transcription as the absence of this highest order structure in the resting state of the chromomeres. Genes that are constantly active in all tissues may dispense with highest order interphase structure and with the corresponding control mechanism, and the fugas involved thus may not puff. Puffs, large chromomeres and highest order interphase euchromatic DNP structure seem to be correlated with genes that need to be transcribed only under certain developmental conditions. It is proposed that the function of high order structure is to sequester genetic material, namely mainly controller sequences. Since such high order structure, in most cases, would be built up to house the controller dependencies of just one structural gene, the amount of DNA per structural gene needed for gene control would be considerable, and the concept, if correct, would go a long way towards explaining the c-value paradox ("excess" DNA in eukaryotes). In eurygenic determination, the high order structure is thought to be conditioned for melting or to actually melt to an intermediate level of structure. From there, stenogenic control, leading to transcription, is considered to carry the melting process further to yet lower structural levels...

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.

The location of 5S (ribosomal) RNA genes in Drosophila hydei

The location of the 5S ribosomal RNA cistrons in band 2-23B1,2 of the polytene (salivary gland) chromosomes of Drosophila hydei was indicated by in situ hybridization of tritiated low molecular weight RNA fractionated from total in vivo synthesized larval RNA or from in vitro synthesized salivary gland RNA and competition of the hybridization of this RNA by 5S RNA obtained from calf lens ribosomes. -- At the submicroscopic level, band 2-23B1,2 in salivary gland chromosomes shows a compact organization. The adjacent region, 23B2, is slightly puffed and displays typical RNP particles, some of which may be observed close to band 2-23B1,2.