Transcription factor DREF regulates expression of the microRNA gene bantam in Drosophila melanogaster

The bantam gene encodes a vital microRNA and has a complex expression pattern in various tissues at different stages of Drosophila development. This microRNA is involved in the control of normal development of the ocular and wing imaginal discs, the central nervous system, and also in maintaining the undifferentiated state of stem cells in the ovaries of adult females. At the cellular level, bantam stimulates cell proliferation and prevents apoptosis. The bantam gene is a target of several conserved signaling cascades, in particular, Hippo. At the moment, at least ten proteins are known to directly regulate the expression of this gene in different tissues of Drosophila. In this study, we found that the bantam regulatory region contains motifs characteristic of binding sites for DREF, a transcription factor that regulates the expression of Hippo cascade genes. Using transgenic lines containing a full-length bantam lethality-rescuing deletion fragment and a fragment with a disrupted DREF binding site, we show that these motifs are functionally significant because their disruption at the bantam locus reduces expression levels in the larvae and ovaries of homozygous flies, which correlates with reduced vitality and fertility. The effect of DREF binding to the promoter region of the bantam gene on its expression level suggests an additional level of complexity in the regulation of expression of this microRNA. A decrease in the number of eggs laid and a shortening of the reproductive period in females when the DREF binding site in the regulatory region of the bantam gene is disrupted suggests that, through bantam, DREF is also involved in the regulation of Drosophila oogenesis.

The study of the regulatory region of the Drosophila melanogaster Notch gene by new methods of directed genome editing

The Notch gene plays a key role in the development of organs and tissues of neuroectodermic origin, including the nervous system. In eukaryotic organisms, the Notch pathway is involved in cell fate determination. The Notch gene was first discovered in Drosophila melanogaster. In mammals, the family of Notch receptors includes four homologues. In humans, mutations in the Notch gene cause several hereditary diseases and carcinogenesis. Studies of the regulatory zone of the Notch gene in D. melanogaster have been conducted for several decades. We review their results and methods. The regulatory zone of the Notch gene is in the region of open chromatin state that corresponds to the 3C6/3C7 interband on the cytological map of polytene chromosomes of D. melanogaster salivary glands. The development of new methods for directed genome editing made it possible to create a system for introducing directed changes into the regulatory zone of the gene. Using the CRISPR/Cas9 system, we obtained a directed 4-kilobase deletion including the 5’-regulatory zone, promoter, and the first exon of the Notch gene and introduced the attP site into the first intron of the Notch gene. This approach enabled targeted changes of the sequence of the regulatory and promoter regions of the gene. Thus, it provided a new powerful tool for studies of Notch gene regulation and the organization of the open chromatin state.

Targeted mutagenesis of Drosophila RNaseZ gene by homologous recombination

For the first time we used a homologous recombination method to obtain complete and precise deletion of Drosophila dRNaseZ gene. In the founder line of flies in which the RNaseZ sequence was replaced by attP site, the full-length sequence of the gene was reintegrated, and its functionality was shown. This approach will allow us to generate further gene mutations in different domains of dRNaseZ protein and discover a broad spectrum and uncover functions outside of tRNA processing.

ECTOPIC TETHERING OF THE CHROMATOR PROTEIN IN UASDBD(GAL4) SYSTEM AS APPROACH FOR STUDYING OF THE INSULATOR PROTEINS IN DROSOPHILA MELANOGASTER POLYTENE CHROMOSOMES

Chromatin insulator proteins are one of the major components that determine the domain organization of the genome. According to the latest data, they can mark the boundaries of topological domains and prevent the spread of silent chromatin to adjacent areas. One approach to the analysis of the actions of these proteins is to use the ectopic involvement in the UAS>DBD(GAL4). The method allows to evaluate the effect of selected protein in chromatin organization, to establish its association with other insulator proteins and influence on the processes of transcription and replication. and influence the processes of transcription and replication. In this study, we have developed and tested the functionality of the system components in ectopic tethering of the Chromator (Chriz) to the region of intercalary heterochromatin 10A1-2. Preliminary data have been obtained showing that ectopically tethered Chromator to the band 10A1-2 can induce a partial decompactization of the band chromatin. Further use of this experimental model provides the opportunity to investigate the effect of insulator proteins on the chromatin structure.

[The influence of morphogene Wg on the formation of an ectopic eye in Drosophila melanogaster]

Due to the ectopic expression of the ey gene in the wing imaginal disc under the action of the 1096-Gal4 driver, a part of the wing disc cells change their fate and become eye cells. Ectopic eyes are induced in definite regions of the wing disc and form a stable pattern on the wing of an adult fly. Here, we have shown that the ectopic expression of Wg inhibits the formation of ectopic eyes, and conversely the expression of Wg is reduced in the sites of ectopic Ey expression. Experiments with overexpression of the vesicular traffic protein H rs capable of inhibiting the Wg signaling agree with the notion on antagonism of Wg and Ey in ectopic eyes. Our results confirm that the processes of formation of normal and ectopic eyes are principally similar with regard to genetic control.

[Functional organization of interbands in Drosophila polytene chromosomes]

The functional organization of particular chromosome regions is tightly associated with their function in eukaryotic cells. Details of this association are among the most topical problems of modem genetics. The paper characterizes the results of recent research of the specifics of the genetic organization and chromatin decondensation in interbands of Drosophila polytene chromosomes. Data on functional heterogeneity of interbands are considered. Experimental findings point to a lack of correlation between the decondensed chromatin state and the observed transcription level in particular interbands. The DNA sequences responsible for the interband formation are principally identifiable via site-specific homologous FRT/FLP recombination between two P transposons contained in chromosomes. The results allow a search for particular protein factors that are involved in the decondensed state of interbands and structural and functional differentiation of polytene chromosomes.

Temperature Sensitive Nanocapsule of Complex Structural Form for Methane Storage

The processes of methane adsorption, storage and desorption by the nanocapsule are investigated with molecular-dynamic modeling method. The specific nanocapsule shape defines its functioning uniqueness: methane is adsorbed under 40 MPa and at normal temperature with further blocking of methane molecules the K@C60(1+) endohedral complex in the nanocapsule by external electric field, the storage is performed under normal external conditions, and methane desorption is performed at 350 K. The methane content in the nanocapsule during storage reaches 11.09 mass%. The nanocapsule consists of tree parts: storage chamber, junction and blocking chamber. The storage chamber comprises the nanotube (20,20). The blocking chamber is a short nanotube (20,20) with three holes. The junction consists of the nanotube (10,10) and nanotube (8,8); moreover, the nanotube (8,8) is connected with the storage chamber and nanotube (10,10) with the blocking chamber. The blocking chamber is opened and closed by the transfer of the K@C(60) (1+) endohedral complex under electrostatic field action.

[Distribution of induced chromosome rearrangement breakpoints along the chromosome length and the problem of intercalary heterochromatin in Drosophila melanogaster]

Historically, the term "intercalary heterochromatin" was based on the finding that induced chromosome rearrangements occur at a higher frequency in the corresponding regions. The available molecular genetic data and, in particular, the results of the Drosophila Genome Project made it possible to decide between two possible explanations of the preferential location of chromosome rearrangement breakpoints in intercalary heterochromatin regions. Namely, a higher frequency of radiation-induced rearrangements in these regions correlates with the DNA content and probably lacks an association with the features of chromatin organization.

Intercalary heterochromatin in polytene chromosomes of Drosophila melanogaster

Intercalary heterochromatin consists of extended chromosomal domains which are interspersed throughout the euchromatin and contain silent genetic material. These domains comprise either clusters of functionally unrelated genes or tandem gene duplications and possibly stretches of noncoding sequences. Strong repression of genetic activity means that intercalary heterochromatin displays properties that are normally attributable to classic pericentric heterochromatin: high compaction, late replication and underreplication in polytene chromosomes, and the presence of heterochromatin-specific proteins. Late replication and underreplication occurs when the suppressor of underreplication protein is present in intercalary heterochromatic regions. Intercalary heterochromatin underreplication in polytene chromosomes results in free double-stranded ends of DNA molecules; ligation of these free ends is the most likely mechanism for ectopic pairing between intercalary heterochromatic and pericentric heterochromatic regions. No support has been found for the view that the frequency of chromosome aberrations is elevated in intercalary heterochromatin.

[Contribution of the SuUR gene to the organization of epigenetically repressed regions of Drosophila melanogaster chromosomes]

A significant portion of a eukaryotic genome is silent (epigenetically repressed). In Drosophila melanogaster, this portion includes mainly regions of pericentric and intercalary heterochromatin and euchromatin regions subject to position-effect variegation. Detailed study of the organization of intercalary heterochromatin regions of Drosophila melanogaster polytene chromosomes started from the discovery of the SuUR gene (Suppressor of UnderReplication). The ability of the SuUR mutation to suppress underreplication in intercalary heterochromatin regions was used for molecular tagging of these regions. We showed that underreplicated intercalary heterochromatin regions contained silent unique genes and retained the features of late replication and transcriptionally inactive chromatin state in various cell types. Over 50% of these regions contain unique genes clustered on the base of coordinated expression. The origin of clusters and putative mechanisms of their gene expression are discussed. Data on the SuUR gene, its expression, and effect on polytene chromosome structure and replication are summarized.

Functional dissection of the Suppressor of UnderReplication protein of Drosophila melanogaster: identification of domains influencing chromosome binding and DNA replication

The Suppressor of UnderReplication (SuUR) gene controls the DNA underreplication in intercalary and pericentric heterochromatin of Drosophila melanogaster salivary gland polytene chromosomes. In the present work, we investigate the functional importance of different regions of the SUUR protein by expressing truncations of the protein in an UAS-GAL4 system. We find that SUUR has at least two separate chromosome-binding regions that are able to recognize intercalary and pericentric heterochromatin specifically. The C-terminal part controls DNA underreplication in intercalary heterochromatin and partially in pericentric heterochromatin regions. The C-terminal half of SUUR suppresses endoreplication when ectopically expressed in the salivary gland. Ectopic expression of the N-terminal fragments of SUUR depletes endogenous SUUR from polytene chromosomes, causes the SuUR- phenotype and induces specific swellings in heterochromatin.

Effect of the Suppressor of Underreplication (SuUR) Gene on Position-Effect Variegation Silencing in Drosophila melanogaster

It has been previously shown that the SuUR gene encodes a protein located in intercalary and pericentromeric heterochromatin in Drosophila melanogaster polytene chromosomes. The SuUR mutation suppresses the formation of ectopic contacts and DNA underreplication in polytene chromosomes; SuUR+ in extra doses enhances the expression of these characters. This study demonstrates that heterochromatin-dependent PEV silencing is also influenced by SuUR. The SuUR protein localizes to chromosome regions compacted as a result of PEV; the SuUR mutation suppresses DNA underreplication arising in regions of polytene chromosomes undergoing PEV. The SuUR mutation also suppresses variegation of both adult morphological characters and chromatin compaction observed in rearranged chromosomes. In contrast, SuUR+ in extra doses and its overexpression enhance variegation. Thus, SuUR affects PEV silencing in a dose-dependent manner. However, its effect is expressed weaker than that of the strong modifier Su(var)2-5.

Ectopic expression of the Suppressor of Underreplication gene inhibits endocycles but not the mitotic cell cycle in Drosophila melanogaster

The Suppressor of Underreplication ( SuUR) gene contributes to the regulation of DNA replication in regions of intercalary heterochromatin in salivary gland polytene chromosomes. In the SuUR mutant these regions complete replication earlier than in wild type and, as a consequence, undergo full polytenization. Here we describe the effects of ectopic expression of SuUR using the GAL4-UAS system. We demonstrate that ectopically expressed SuUR exerts qualitatively distinct influences on polyploid and diploid tissues. Ectopic expression of SuUR inhibits DNA replication in polytene salivary gland nuclei, and reduces the degree of amplification of chorion protein genes that occurs in the follicle cell lineage. Effects caused by ectopic SuUR in diploid tissues vary considerably; there is no obvious effect on eye formation, but apoptosis is observed in the wing disc, and wing shape is distorted. The effect of ectopic SuUR expression is enhanced by mutations in the genes E2F and mus209 ( PCNA). Differential responses of polyploid and diploid cells to ectopic SuUR may reflect differences in the mechanisms underlying mitotic cell cycles and endocycles.

Influence of the SuUR gene on intercalary heterochromatin in Drosophila melanogaster polytene chromosomes

Salivary gland polytene chromosomes of Drosophila melanogaster have a reproducible set of intercalary heterochromatin (IH) sites, characterized by late DNA replication, underreplicated DNA, breaks and frequent ectopic contacts. The SuUR mutation has been shown to suppress underreplication, and wild-type SuUR protein is found at late-replicating IH sites and in pericentric heterochromatin. Here we show that the SuUR gene influences all four IH features. The SuUR mutation leads to earlier completion of DNA replication. Using transgenic strains with two, four or six additional SuUR(+) doses (4-8xSuUR(+)) we show that wild-type SuUR is an enhancer of DNA underreplication, causing many late-replicating sites to become underreplicated. We map the underreplication sites and show that their number increases from 58 in normal strains (2xSuUR(+)) to 161 in 4-8xSuUR(+) strains. In one of these new sites (1AB) DNA polytenization decreases from 100% in the wild type to 51%-85% in the 4xSuUR (+) strain. In the 4xSuUR(+) strain, 60% of the weak points coincide with the localization of Polycomb group (PcG) proteins. At the IH region 89E1-4 (the Bithorax complex), a typical underreplication site, the degree of underreplication increases with four doses of SuUR(+) but the extent of the underreplicated region is the same as in wild type and corresponds to the region containing PcG binding sites. We conclude that the polytene chromosome regions known as IH are binding sites for SuUR protein and in many cases PcG silencing proteins. We propose that these stable silenced regions are late replicated and, in the presence of SuUR protein, become underreplicated.

The Drosophila suppressor of underreplication protein binds to late-replicating regions of polytene chromosomes

In many late-replicating euchromatic regions of salivary gland polytene chromosomes, DNA is underrepresented. A mutation in the SuUR gene suppresses underreplication and leads to normal levels of DNA polytenization in these regions. We identified the SuUR gene and determined its structure. In the SuUR mutant stock a 6-kb insertion was found in the fourth exon of the gene. A single SuUR transcript is present at all stages of Drosophila development and is most abundant in adult females and embryos. The SuUR gene encodes a protein of 962 amino acids whose putative sequence is similar to the N-terminal part of SNF2/SWI2 proteins. Staining of salivary gland polytene chromosomes with antibodies directed against the SuUR protein shows that the protein is localized mainly in late-replicating regions and in regions of intercalary and pericentric heterochromatin.

[Effect of four doses of the Su(UR)ES gene on intercalary heterochromatin in Drosophila melanogaster]

Polytene chromosomes of salivary glands of various Drosophila melanogaster strains containing two doses of the normal Su(UR)ES allele have a constant set of intercalary heterochromatin (IHC) sites. Their DNA is underreplicated, which leads to breaks and ectopic contacts emerging at a certain rate. Almost no underreplication, breaks, or ectopic conjugation are present in mutants lacking the normal Su(UR)ES gene product. It could be expected that an increase in the number of the Su(UR)ES+ gene doses would, in turn, drastically increase ectopic conjugation and breakage. To test this hypothesis, a strain of D. melanogaster was obtained with two additional doses of Su(UR)ES+ introduced into its genome. The flies with four gene doses exhibited a considerable increase in ectopic conjugation: both the proportion of regions participating in conjugation and the number of chromosomes with numerous contact nodes were increased. As a result, chromosomes that were straight and well-stretched in homozygotes for the mutation in Su(UR)ES became twisted and wound and contained many loops or nodes. Many chromosomes were wound too tightly for cytological analysis. Four doses of Su(UR)ES+ considerably increased the number of weak "points." For example, the 2R chromosome has only 3 weak points in strains with two doses of Su(UR)ES+ and as many as 22 weak points in the strain with four doses. In the transgenic strain, the frequency of breaks in previously known weak points increased, and new breaks appeared in 19 additional sites. All new break points appeared in the regions that were earlier described as regions of late replication in the S phase.

[Age-related changes in crossing over in Drosophila resemble the picture of interchromosomal effect of chromosome rearrangement on crossing over]

Crossing over in the left arm of chromosome 2 (2L) was studied in successive broods of Drosophila melanogaster females carrying intact chromosomes (+/+), inversion Muller-5 in the X chromosome (M-5/+), and insertion of the Y-chromosome material into region 34A (Is(2L)/+). The regions net-dp, dp-b, b-pr and pr-cn were examined in 14 two-day-old broods of females +/+ and M-5/+ and in 10 broods of females Is(2L)/+. In all lines, the highest level of crossing over was in the first three broods (eggs laid during the first 6 days of oviposition) and the lowest level in the broods 7-8 (eggs laid at days 14-16). A high rate of crossing over in the first broods of females +/+ and M-5/+ was due to an increment of exchanges in the proximal euchromatin regions (b-pr and pr-cn) and to an increase in the number of tetrads with double exchanges. These changes are similar to a pattern of the interchromosomal effect on crossing over (IEC) in structurally normal chromosomes. In Is(2L)/+ females, a high level of crossing over was due to extensive exchanges in the interstitial regions net-dp and dp and an increase in the number of tetrads with single exchanges. These changes resembled the IEC in rearranged chromosomes (in this case, in chromosomes bearing an insertion). Thus, the age changes of crossing over are similar to the consequences of the presence or absence of IEC. Age changes in crossing over in a chromosome depended both on the local rearrangements in this chromosome (the local effect on crossing over, LEC) and on rearrangements in nonhomologous chromosomes (IEC). In the first broods, both LEC and IEC decreased with an increase in the level of crossing over. In subsequent broods, the reduced level of crossing over was accompanied by an increase in both LEC and IEC. This suggests that the mechanisms responsible for the age changes in crossing over and IEC may have common steps. The contact model of crossing over may explain the similarity between the age changes in crossing-over and IEC. It is suggested that both phenomena result from delayed determination of crossing over in a meiotic cell. This may occur due to the retarded formation of the local contacts in one of the homologous chromosome pairs or because a higher number of local contacts is required to trigger crossing over in a meiotic cell (of early age).

[Microcloning and characteristics of DNA from regions of the centromeric heterochromatin of Drosophila melanogaster polytene chromosomes]

A method of microcloning, which involves microsurgical excision of chromosome fragments, DNA amplification by means of a polymerase chain reaction (PCR), and ligation of amplified products with plasmids, was employed in studying Drosophila polytene chromosomes for the first time. Clones of the DNA library thus obtained contained inserts varying in size from 0.1 to 0.5 kb. DNA sequencing of five clones of the library showed that pericentromeric heterochromatin contained the 17.6 and 297 retrotransposons, the ninja retrotransposon characteristic of D. simulans, and two Drosophila repetitive elements, a8 and a12, the function of which remains unknown.

Su(UR)ES: a gene suppressing DNA underreplication in intercalary and pericentric heterochromatin of Drosophila melanogaster polytene chromosomes

A genetic locus suppressing DNA underreplication in intercalary heterochromatin (IH) and pericentric heterochromatin (PH) of the polytene chromosomes of Drosophila melanogaster salivary glands, has been described. Found in the In(1)scV2 strain, the mutation, designated as Su(UR)ES, was located on chromosome 3L at position 34. 8 and cytologically mapped to region 68A3-B4. A cytological phenotype was observed in the salivary gland chromosomes of larvae homozygous and hemizygous for Su(UR)ES: (i) in the IH regions, that normally are incompletely polytenized and so they often break to form "weak points," underreplication is suppressed, breaks and ectopic contacts disappear; (ii) the degree of polytenization in PH grows higher. That is why the regions in chromosome arm basements, normally beta-heterochromatic, acquire a distinct banding pattern, i. e., become euchromatic by morphological criteria; (iii) an additional bulk of polytenized material arises between the arms of chromosome 3 to form a fragment with a typical banding pattern. Chromosome 2 PH reveals additional alpha-heterochromatin. Su(UR)ES does not affect the viability, fertility, or morphological characters of the imago, and has semidominant expression in the heterozygote and distinct maternal effect. The results obtained provide evidence that the processes leading to DNA underreplication in IH and PH are affected by the same genetic mechanism.

[Search for insertion mutations disrupting mitosis using a transposon from the reporter gene in Drosophila melanogaster]

Transpositions of the vector P[lArB] into the regions 78D, 61F, and 85F of chromosome 3, which result in various anomalies of mitoses in neural ganglions of homozygous larvae, were obtained by insertion mutagenesis. The tissue specificity of regulatory elements controlling the reporter gene was studied by staining for the activity of beta-galactosidase reporter gene of the vector P[lArB]. These regulatory elements are suggested to be the enhancers of the genes carrying insertions. In all studied mutants, staining for beta-galactosidase was found in tissues containing actively proliferating cells. The staining of germarium in adult female ovaries was the most pronounced. The germarium staining pattern was used for the identification of novel insertions leading to mitosis abnormalities. The P1003 (99F) insertion was found, which according to preliminary data leads to an increase in the mitotic index and anomalies of chromosome structure in neuroblasts of homozygous larvae. In addition, the 22w (42A) insertion leading to chromosome arrest in metaphase was found.

[Differences in the genetic control of the formation of induced chromosomal aberrations and mitotic exchanges]

The maternal effect of the mutation mus104D1 (defect of postreplication repair) on the yield of translocations involving heterochromatin breaks induced by irradiation of mature spermatozoa has been investigated. The mutation has been shown to bring about a threefold increase in the frequency of translocations. No significant maternal effect of this mutation on the frequency of mitotic exchanges induced by irradiation of mature hyperploid spermatozoa has been observed. This is in agreement with published evidence on properties of the mei41 mutation (allete mus104D1). However, our results are more accurate as both processes--formation of translocations and mitotic exchanges--were studied in the same tissue and at the same stage of development.

[Directed search for streptomycetes that are producers of ionophoric antibiotics]

Streptomyces producing ionophore antibiotics were searched for in various natural substrata, such as soil, sea water, and coastline sea mud, in many geographical regions. A total of 393 Streptomyces strains were isolated, 308 of which proved to be capable of producing antibiotics; 135 of these contained water-insoluble antibiotically active substances in the mycelium. Studies by radioindication, spectrophotometry, and potentiometry demonstrated that these partially purified hydrophobic antibiotics possessed ionophore properties. Selectivity to monovalent potassium and ammonium cations seems to be a characteristic feature of hydrophobic antibiotics derived from Streptomyces differing them from other bacteria.

[Increased mitotic crossing over of heterochromatin in the first cleavage division of Drosophila melanogaster]

It is shown that, although no compaction of paracentromeric heterochromatin occurs during the first cleavage division in Drosophila melanogaster, the frequency of mitotic crossing-over in corresponding chromosome regions is increased, as compared to that in euchromatin. Because a similar situation is observed at later stages of Drosophila development, at which compact chromatin regions become well-manifested, it is concluded that the effect of heterochromatin on the frequency of crossing-over does not depend on its packing. A positive correlation between crossing-over events in paracentromeric heterochromatin and euchromatin was observed. This effect is probably due to the formation of a continuous region of somatic synapsis, which facilitates the process of mitotic crossing-over. On this basis, it is proposed that the effect of heterochromatin on mitotic crossing-over is associated with preferential chromosome pairing in the corresponding regions.

[The mechanism of the interchromsomal effect on crossing over in Drosophila melanogaster: delayed crossing over]

Interchromosomal effect on crossing-over (IEC) in autosome 2 has been studied in 2/F(2L); F(2R) females heterozygous for free arms (acrocentrics) and in Is(Y;2)419/+ females with an insertion of Y-material into the region 34A. IEC was induced by In(1)dl-49 + BM1 inversion. Manifestations of IEC included increased recombinational length of chromosome 2 and decreased interference. IEC was not observed in Df(2L)TW161/+ females with 38A-40 deletion. The patterns of IEC in three types of gametes of the 2/F(2L); F(2R) female depended on the pairing relations of the affected chromosome (chromosome-responder). In the case of normal pairing between the metacentric autosome 2 (the metacentric) and the F(2R) acrocentric, the increment in 2R length was minimal (20%), and the increment in the proportion of multiple-exchange (high-rank) tetrads (E2 + E3), maximal (8 to 10%). In the case of disturbed pairing 2-F(2R) nondisjunction, 2R length was increased by 77%, paralleled by a minimal increase in the proportion of high-rank tetrads (4%). Similarly, in females with the insertion, a pronounced increase in 2L length (74%) was associated with a moderate level of high-rank tetrads. When pairing in the chromosome-responder was normal, the increment in crossing-over was maximal in the pericentromeric region. In the case of disturbed pairing, this maximum either shifted toward the middle of the arm 2-F(2R) nondisjunction, or occupied a distal position (in females with the insertion). It is concluded that IEC pattern depends on the order of pairing in the chromosome-responder. The mechanism of IEC appears to be related to pairing "defects" within the responder. It is tempting to speculate that the onset of crossing-over is a whole-cell event, which is regulated by the overall level of chromosome pairing within the meiotic cell. Chromosomal aberrations increase the time required for attaining this level, and the start of crossing-over is delayed. As a result, (1) exchanges are observed in the regions of late synapsis, which are usually not involved in crossing-over; (2) overabundance of recombination enzymes, caused by delayed start of crossing-over, creates the conditions for decreased interference in paired regions.

[The nature of radiation-induced exchanges between homologous chromosomes in the hyperploid drosophila male pronucleus]

Exchanges between nondisjoined male chromosomes induced in mature sperm are reciprocal (all the broken chromosome ends reunite). These exchanges are single-impact events; a break in a chromosome induces a break in the homologous site of its homologue.

[The role of heterochromatin in meiotic pairing of chromosomes in Drosophila melanogaster males]

The gamete spectrum of males containing metacentric chromosome 2 and combinations of 2R and 2L acrocentrics with heterochromatic regions of different size was studied. Changes in the size of the heterochromatic block located to the right of the centromere in the acrocentric 2L can alter the disjunction of this acrocentric, as well as that of the acrocentric of the opposite 2R arm. It is therefore concluded that, in addition to the major pairing site located at the base of arm 2L, the sites located in heterochromatin of the 2R arm also function during meiotic pairing in males. An account is given of the participation of a 2L acrocentric, containing Y-material at the telomeric end, either in pairing with metacentric chromosome 2 or with the X-chromosome, but not with both of them simultaneously. This phenomenon may be due to the spatial relations of pairing sites and the coiling of pairing chromosomes.

[Radiation-induced mitotic crossing-over in the pronucleus of Drosophila males]

Exchanges in nondisjoined homologous chromosomes of irradiated mature spermatozoa were obtained. Exchanges absent in the control experiment where one of those chromosomes was irradiated in mature sperm and the other (non-irradiated) comes from female. Those results coupled with Abeleva et. al. data show that the exchanges observed were formed in male pronucleus.

[Lipoprotein (A) in patients with coronary atherosclerosis]

The serum level of lipoprotein (a) (LP(a] was measured in 117 patients with coronary heart disease (CHD) angiographically documented, 22 patients with unaltered coronary arteries as evidenced by coronary angiography (Control Group 1), and 28 subjects without clinical CHD signs (Control Group 2). Total cholesterol, triglycerides, low and high density lipoprotein cholesterol, and apolipoproteins (apo) AI and B were measured in the patients and healthy subjects. They were found to be higher in CHD patients than in patients with unaltered arteries and healthy subjects. The level of LP(a) was significantly increased in patients with three-vessel disease as compared to the control groups and patients with single-vessel disease. There was no relation of LP(a) to total cholesterol, triglyceride, high density lipoprotein cholesterol, low and very low density lipoprotein cholesterol, apo-AI, apo-B, age, and sex. The findings supports the assumption that LP(a) is associated with coronary atherosclerosis.

[Use of Streptomyces sp. 26-115 protoplasts inactivated by heating in fusion experiments]

When heated at 55 degrees C for 30 or 60 minutes protoplasts of auxotrophic mutants of Streptomyces sp. 26-115 producer of actinomycin C (active and inactive variants) lost their capacity for regeneration. The protoplasts heated at at 55 degrees C for 30 minutes and not for 60 minutes maintained some ability to yield recombinants on fusion under the effect of PEG 6000. Unlike the parent active strain, the colonies formed by the spores of the prototrophs yielding on fusion of the intact protoplasts showed wide ranges of antibiotic activity against M. flavus while a significant part of the colonies was inactive. The use of the inactive variant protoplasts heated at 55 degrees C for 30 minutes in the fusion procedure increased the proportion of the inactive variants.

[Reduction of actinomycin biosynthesis during protoplast regeneration in an inactive variant producer]

During regeneration of protoplasts in the inactive variant H-2 of the actinomycin-producing organism Streptomyces sp. 26-115 there were detected 1-4 per cent of the colonies synthesizing the antibiotic. The frequency of such colonies (H-2R) did not increase after exposure of the H-2 protoplasts to the fusing agent PEG-1000. The population grown from one colony after three passages on pea agar was sufficiently homogeneous by the antibiotic production property. Variant H-2R was more stable to the effect of streptomycin than the initial variant H-2.