[Intercalary heterochromatin in the genome of Drosophila].

The modern concept of intercalary heterochromatin as polytene chromosome regions exhibiting a number of specific characteristics is formulated. DNA constituting these regions is replicated late in the S period; therefore, some strands of polytene chromosomes are underrepresented; i.e., they are underreplicated. Late-replicating regions account for about 7% of the genome; genes are located there in clusters of as many as 40. In general, the gene density in the clusters is substantially lower than in the main part of the genome. Late-replicating regions have an inactivating capacity: genes incorporated into these regions as parts of transposons are inactivated with a higher probability. These regions contain a specific protein SUUR affecting the rate of replication completion.

[Maintenance of the patterns of expression of homeotic genes in the development of Drosophila melanogaster by proteins of the polycomb, trithorax, and ETP groups].

Proteins encoded by genes of the groups Polycomb (PcG), trithorax (trxG), and the Enhancer of Trithorax and Polycomb Group (ETP) are important regulators of expression of most developmental genes. Data concerning all currently described genes assigned to these groups are summarized in the review. Genetic interactions of these genes and phenotypical manifestation of their mutations are described. Data on the PcG, trxG, and ETP proteins are systemized. Questions are considered concerning the formation of multimeric complexes containing proteins of these groups, recruitment of these complexes to regulatory elements of target genes, and the mechanisms of activation/repression of gene expression.

[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.

[Molecular genetic analysis of Thrithorax-like gene encoded transcriptional factor GAGA in Drosophila melanogaster]. / Molekuliarno-geneticheskii analiz gena Trithorax-like, kodiruiushchego transkriptsionnyi faktor GAGA Drosophila melanogaster.

The Drosophila melanogaster Trithorax-like (Trl) gene is classed with the trx-G genes and codes for several isoforms of the GAGA transcription factor (GAF) which regulates expression of homeotic and numerous other genes. GAF acts as a transcriptional antirepressor, i.e., its interaction with nucleosomal DNA results in the open chromatin conformation in promoter gene regions. The regions thereby become accessible to other transcription factors. As mutations of the Trl gene enhance position effect variegation and disturb chromosome segregation in mitosis and meiosis, GAF is thought to play another, more significant role in determining the chromatin structure. To study the molecular basis of its pleiotropic effect, the Trl gene was subjected to a structural analysis. The genomic Trl gene was sequenced, the sizes of its exons and introns was established, and a complex structure of the 5' and 3' gene regions was demonstrated. The Trl13C, Trl62, DfTrlR67, and DfTrlR85 mutations were exactly mapped. In addition, four insertions of the P element were identified as Trl alleles (Trll(3)s2325, TrlEP(3)3184, TrlEP(3)3191, and TrlEP(3)3609). The viability at various developmental stages was studied in homozygotes for the Trl mutations and in interallelic compounds. The following lethality stages were established: hatching, (Trl13C, DfTrlR85, TrlEP(3)3609), larval molts (Trll(3)s2325), pupation, metamorphosis (DfTrlR67, Trl62), and eclosion (several compounds).