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Molecular Cytogenetics Laboratory


Head I.F.Zhimulev, Corresponding Member of the RAS
zhimulev@bionet.nsc.ru


Insertion of the P-transposon into the interband results in the formation of a new band (arrow).
a - map of the region, b - the region in normal srain, c - the same region containing transposon insertion

Distribution of antibodies raised against proteins involved in the dosage compensation mechanism in males of Drosophila

A scheme of molecular-genetic organization of the 10A1-2 band of the D. melanogaster polytene X chromosome.
a - the genes discovered using mutagenesis; b - the genes discovered using mRNA localization; c - repeated sequences of DNA; d - vertical lines with small flags indicate break points of chromsome rearrangement, the arrow of a flag indicates the direction of the rearrangement; e - bands (9F10-10A8) of the region studied; f - map of the DNA (from 0 through 300 kb), arrows indicate points of "evolutionary breaks" in the chromosomes of related species D. virilis, D. repleta, D. hydei

A fragment of a Drosophila melanogaster polytene chromosome under the electron microscope. Dark and light stripes represent bands and interbands, respectively

 

a, b - weak points in the 71C band in a polytene chromosome of wild type, c - full polytenization of the region in Su(UR)ES strain

Characteristics of the DNA sequences in the 61C7/8 interband.
Red triangle - site of the P-element insertion; NR - region of nucleosome binding; MAR - matrix-binding site; B, S, R, T, H, P - site of different restriciton enzyme cleavage; T, C - directions to the telomere and centromere, respectively


The major focus in research is the structural and functional organization of the Drosophila polytene chromosomes, the most advantageous model of the interphase chromosome.

The objective is to clarify the functional role of the morphological elements of the chromosome: the band (the chromomere), the interband, the puff, and the heterochromatin regions. A model for the dynamic organization of the interphase chromosome was suggested and based on experimental work demonstrating its validity. The idea underlying the model contradicts the previously accepted. The model implies that the morphology of the region is determined by the level of its transcriptional activity, the bands may contain from one to dozens of functionally unrelated genes.

Evidence for the transition of the euchromatic regions into heterochromatic condition as a result of their inactivation under position effect variegation (displacement to heterochromatin) were obtained when verifying the model. Heterochromatin is able to extend its silent state to hundreds of megabases of DNA. Reverse transition, the transformation of heterochromatin regions into morphologically euchromatic under the effect of genetic and enviromental factors and chromosome rearrangements, was also demonstrated. Recently, a new Su(UR)ES gene was identified at the laboratory. Mutations in the gene suppress DNA underreplication both in pericentric and intercalary heterochromatin. In Su(UR)ES mutant chromosomes, breaks disappear in intercalary heterochromatin, and regions with banding pattern appear in pericentric heterochromatin. This was evidence that the genetic control of the "silent" heterochromatic regions with different informational content is similar.

A new approach to study of the interband organization was suggested and applied at the laboratory. The approach is based on transposon insertions into the interbands, and transposon DNA is used as a probe for isolating interband DNA. In this way, sequences of four interbands were obtained and analysed. They are represented by unique DNA, enriched with AT-repeats and nuclear matrix binding sites, they have no extensive reading frames and are evolutionary nonconserved.

The chromosome proteins involved in band and interband organization are presently under study by the method of indirect fluorescence.

Regarding the puffs, the most active chromosome regions, of significance was the finding of the complex genetic locus BR-C that plays the key role in Drosophila metamorphosis. It is the earliest step in the complex cascade of changes in the chromosome transcriptional activity produced by hormone ecdysterone. This was the first genetic evidence for the cascade pattern of hormonally induced development.

The mutations and results obtained in the laboratory are used worldwide to study development regulation in Drosophila.