Cytogenetic fine structure and chromosomal localization of the white gene in Drosophila melanogaster

The Drosophila mus101 gene, which links DNA repair, replication and condensation of heterochromatin in mitosis, encodes a protein with seven BRCA1 C-terminus domains

The mutagen-sensitive-101 (mus101) gene of Drosophila melanogaster was first identified 25 years ago through mutations conferring larval hypersensitivity to DNA-damaging agents. Other alleles of mus101 causing different phenotypes were later isolated: a female sterile allele results in a defect in a tissue-specific form of DNA synthesis (chorion gene amplification) and lethal alleles cause mitotic chromosome instability that can be observed genetically and cytologically. The latter phenotype presents as a striking failure of mitotic chromosomes of larval neuroblasts to undergo condensation of pericentric heterochromatic regions, as we show for a newly described mutant carrying lethal allele mus101(lcd). To gain further insight into the function of the Mus101 protein we have molecularly cloned the gene using a positional cloning strategy. We report here that mus101 encodes a member of the BRCT (BRCA1 C terminus) domain superfamily of proteins implicated in DNA repair and cell cycle checkpoint control. Mus101, which contains seven BRCT domains distributed throughout its length, is most similar to human TopBP1, a protein identified through its in vitro association with DNA topoisomerase IIbeta. Mus101 also shares sequence similarity with the fission yeast Rad4/Cut5 protein required for repair, replication, and checkpoint control, suggesting that the two proteins may be functional homologs.

Electron microscopic in situ hybridization of digoxigenin-dUTP-labelled DNA probes with Drosophila melanogaster polytene chromosomes

We report a simplified method of electron microscopic (EM) in situ hybridization for standard squashes of Drosophila melanogaster polytene chromosomes using digoxigenin-11-dUTP labelled DNA probes. The method is efficient and reproducible: its high resolution and specificity were demonstrated for the transformed strain 148, in which the insertion was localized precisely as a new thin band both by conventional EM and according to our method. In addition, the method was applied to the fine mapping of the developmentally regulated gene muscle-blind (mbl). On the one hand, mbl was shown to cover the 54B1-2 large band and the adjacent interbands in the 2R polytene chromosome. On the other hand, the use of distantly located DNA probes in the mbl gene allowed us to orientate the transcription unit in the chromosome.

Homeosis and the interaction of zeste and white in Drosophila

Transvection effects in Drosophila melanogaster suggest a form of gene modulation that is responsive to the proximity of homologous genes. These effects have been well characterized at bithorax and decapentaplegic, and in the interaction between the zeste and white genes. The mechanistic basis for transvection is not known. As part of a genetic analysis of transvection, a study is being made of a class of mutations defined as modifiers of the eye color resulting from the interaction of zeste and white. This report details the observations that several of these mutations also have homeotic effects.

Electron microscopical analysis of Drosophila polytene chromosomes. V. Characteristics of structures formed by transposed DNA segments of mobile elements

An electron microscopical (EM) analysis was performed on regions of polytene chromosomes which contained DNA segments of different genetic composition, inserted by P element-mediated transformation into the Drosophila melanogaster genome. In seven of ten regions examined, containing insertions of the hsp28-ry, hsp70-Adh, ryhsp 70-beta-gal genes and of the ry gene tetramer, new bands appeared. Lack of new bands in three other strains is apparently connected with the fusion of the inserted material to preexisting bands. The new bands do not differ morphologically from the usual bands of polytene chromosomes, and their formation is likely due to predominant insertion of DNA segments into interbands. Among the constructs examined, the minimal length of a DNA segment which appears as a new band is about 5 kb; the DNA packing ratio in the new bands varies from 30 to 50. Activation of the inserted genes by heat shock has enabled us to observe the puffing characteristics of new bands. A sequence of some one kb forms a large interband, or micropuff; the puff size is correlated with the length of the genes being activated. If a DNA segment contains a single gene, then its activation causes the decompaction of the whole band; however, when a DNA segment consists of two genes and the promoter element of the activated gene is positioned in the middle of the sequence, the band splits and only part is decompacted and puffed. The DNA packing ratio in the puffs is 1.4-3.5. The subsequent deletion of the hsp70 promoter but retention of 23, 59, and 73 by from the transcription start points leads to failure of puff formation. In all the transformed sites an increase in the total length of the interbands adjacent to the insert as compared with the initial interband was observed. This increase appears to be due to decompaction of the P element DAN flanking the inserted segments. It is shown that a DNA segment, consisting of four tandemly repeated ry gene copies and interspersed by material which includes P DNA, forms a complex of loose chromatin in which, however, four bands can be resolved. We also observed a lengthening of interband regions containing only the P element sequence itself. Insertion of the complete 2.9 kb P element into the large single 10A1-2 bound of the X chromosome (an insertion in the region approximately 10 kb to the right of the v gene) causes splitting of the band into two parts and formation of a new interband.(ABSTRACT TRUNCATED AT 400 WORDS)

The nature of radiation-induced mutations at the white locus of Drosophila melanogaster

X-Ray- and neutron-induced mutations at the white locus of Drosophila melanogaster were used to study the nature of radiation-induced genetic damage. Genetic analysis showed the presence of multi-locus deficiencies in 15 out of 31 X-ray mutants and in 26 out of 35 mutants induced by neutrons. The DNA from 11 X-ray and 4 neutron mutants, which were not multi-locus deficiencies, was analyzed by Southern blot-hybridization. Deletions were observed in 2 X-ray and 1 neutron mutant. In combination with cytogenetic techniques, chromosomal rearrangements affecting the white locus (translocations, inversions, etc.) were identified in 3 X-ray and in 2 neutron mutants. A hot-spot for translocation breakpoints was identified in the left arm of the third chromosome. 5 X-ray mutants, which apparently did not contain large deletions, were subjected to further analysis by the nuclease S1 protection method, after cloning of the white gene. In 4 mutants a small deletion could indeed be detected in this way. Thus it seems that by far the main part of X-ray- and neutron-induced white mutants have arisen through large changes in the white gene, especially deletions.

DNA sequence divergence in the Drosophila virilis group

DNA sequence divergence was analyzed in some sibling species of the Drosophila virilis group. Clones comprising about 0.1% of the genome DNA were selected at random from a D. virilis library for a comparative study on DNA from D. lummei, D. novamexicana, D. borealis, and D. lacicola. Blot hybridization experiments indicated that about 70% of DNA from D. lummei and D. novamexicana and less than 50% of DNA from D. borealis and D. lacicola share sequences that are homologous to DNA in D. virilis. This finding is in excellent agreement with the genealogical tree based on cytological studies (Throckmorton 1982). - Four plasmids with inserts which are present in one or a few copies per genome were hybridized in situ to polytene chromosomes. These experiments demonstrate that (1) homologous "unique" DNA sequences are localized exclusively in homologous bands and (2) homologous bands that appear to be identical in different species may contain different DNA sequences.

The fourth chromosome of Chironomus tentans Malpighian tubules: an ultrastructural study

Morphology and banding pattern of the 4th chromosome in Chironomus tentans Malpighian tubules have been investigated by electron microscopy, using the squash and selection technique. The map we composed from our observations shows a remarkable increase (75%) in band numbers as compared to the map previously presented by Beermann for the 4th chromosome from salivary glands. Extrapolation of this increase to the entire genome would result in a total band number of about 3,500. The mean DNA content of bands can thus be calculated to be about 50 kb. Many bands show a complex structure, including the BR2 band. Some bands seem to result from fusion of smaller components. "Minibands" have also been observed. Some interbands contain RNP particles. In our material the interbands appeared to be made up of fibrils with a diameter of about 120 A. On the basis of these results we estimate the DNA in the interbands as amounting to 2% of the entire genome. The results are discussed with respect to the organization of the polytene chromosomes and the functional significance of the banding pattern.

Mutants affecting position-effect heterochromatinization in Drosophila melanogaster

The dominant suppressor Su(var)b101 and the dominant enhancer En(var)c101 were found to affect significantly white variegation in a strongly variegating line of the Wm4 chromosome (Wm4h) which has been used as standard rearrangement for a genetic dissection of position-effect variegation (Reuter and Wolff, 1981). Both mutations were also shown to affect position-effect heterochromatisation in T (1 ; 4)Wm258-21 and variegation in all the rearrangements tested (white, brown, scute and bobbed variegation). These results suggest that the genes identified encode functions essential for the manifestation of gene inactivation in position-effect rearrangements. It seems also reasonable to assume that in all the rearrangements tested identical heterochromatisation processes lead to inactivation of the genes whose phenotype is variegated.

Physical map of the white locus of Drosophila melanogaster

The white locus of Drosophila melanogaster is a genetically well-characterized locus, mutations in which alter the degree of pattern of pigmentation of the eyes. Using a previously cloned DNA segment containing a portion of the white locus of a mutant allele, we have cloned and characterized the DNA of a 48-kilobase chromosomal region of the Canton S wild-type strain. We have mapped the positions, relative to restriction endonuclease cleavage sites, of several previously characterized chromosomal rearrangement breakpoints that bracket the while locus. These results define a segment of 14 kilobase that contains all of the white locus sequences necessary for the production of a wild-type eye color phenotype. By conventional criteria, no repetitive sequences are present within this 14-kilobase segment; however, we have identified an extremely weak DNA sequence homology between a portion of this segment and a chromosomal region in the vicinity of the zeste locus.

The recombinational analysis of aberrations and the position of the notch locus on the polytene chromosome of Drosophila

The recombinational analysis of heterozygotes for a point-mutant N and a deficiency N suggests that the map region approximated by the interval fa to nd2 is at the right edge of salivary band 3C7 or in the interband to the right. The map region N55ell to fa can be anywhere between the left-interband and the right edge of 3C7. We discovered that small inversions also can be used in the recombinational analysis, and the inversion data support the conclusions already described. The reactivation of latent mutability in a Notch inversion resulted in reinversion of the original aberration, followed by reversion of N to N+. From the same Notch inversion, we isolated a spontaneous deficiency superimposed upon the original aberration, which supported our hypothesis that two of our w to N deficiencies probably originated as deficiencies superimposed upon inversions.

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.

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...

Evaluation and re-evaluation of genetic radiation hazards in man. I. Interspecific comparison of estimates of mutation rates

A detailed presentation is made of the experimental data from the various systems used by Abrahamson et al. [2] to conclude that the per locus per rad (low LET) radiation-induced forward mutation rates in organisms, whose DNA content varies by a factor of about 1000, is proportional to genome size. Additional information pertinent in this context is also reviewed. It is emphasized that the mutation rates cited by Abrahamson et al. [2], although considered as pertaining to mutations at specific loci, actually derive from a broad variety of genetic end-points. It is argued that an initial (if not sufficient) condition for sound inter-specific mutation rate comparisions, covering a wide range of organisms and detecting systems of various sensitivities, requires a reasonalbly consistent biological definition of a specific locus mutation, namely, a transmissible intra-locus change. Granting the differences between systems in their resolving power to detect intragenic change, the data cited in this paper do not support the existence of a simple proportionality between radiotion-induced intra-locus mutation rate and genome size for the different species reviewed here. Furthermore, in Drosophila melanogaster, where individual salivary gland chromosome bands (that can differ greatly in DNA content) are usually associated with individual loci or at least distinct complementation groups, radiation-induced intra-locus mutation rates are not correlated with apparent differences in the DNA content of bands. This result is incompatible with the notion that most of the DNA in a band represents a radiation-mutable target capable of eliciting the kind of mutation observed in mutation rate experiments. All these considerations argue against the validity of the hypothesis of Abrahamson et al. [2] and their generalization that, for the evaluation of genetic radiation hazards in man, we can now "extrapolate from mutation rates obtained in lower organisms to man with greater confidence" on the basis of DNA content (italics are ours).

Electron microscopic studies on the polytene chromosomes of Chironomus thummi salivary glands. I. Ultrastructural mapping

The method of ultrathin sections of unsquashed salivary gland polytene chromosomes of Ch. thummi was applied to their ultrastrual mapping. There was a good agreement between electron micrographs and Hägele's light microscopic map (1970) with respect to the pattern and number of bands. 94% of bands were identified in larval and prepupal chromosomes. In Ch. thummi, band thickness varied from 0.05-0.5 mum. Most characteristic were 0.2-0.3 mum bands. Morphologically, bands were classified as: continuous (frequently with holes and gaps), discrete, dotted and continuous-discrete, discrete-dotted. Band morphology is related to band size, such that smaller bands, as a rule, were also dotted. Bands beginning to puff likewise became dotted. Interbands in unsquashed chromosome sections were from 0.05-0.15 mum. The smallest interbands contained only fibrils, in the larger interbands few granules could be observed. This makes interbands distinguishable from a typical puff with many such granules.

Proposals to the problem of structural and functional organization of polytene chromosomes

The concept of the structural and functional organization of polytene chromosomes is formulated. It is based on the following:Interbands are transcriptional active chromosome regions; they seem to be responsible for the base metabolism of the cell. The stability of these metabolic processes in the cells of different organs results in constancy of chromosome banding pattern. Thus, each functional state of the cell possesses its own banding pattern which may vary depending on activity level and degree of condensation of interbands.The band is considered not as a structural or functional unit, but as a region of the chromosome not involved in transcription at a certain point of time; a band may contain one or several genes.