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Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year 1881 by Balbiani in salivary glands, intestine, muscles, malpighian tubules, and hypoderm of larvae Chironomus plumosus. Hence, these are also called "Salivary gland chromosomes." These are found in insects of the order Diptera and Collembola; in certain organs of mammals; and synergids, antipodes of flowering plants. Polytene chromosomes are also...
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The histone proteins have a flexible N-terminal tail extending out from the nucleosome. These histone tails are often subjected to post-translational modifications such as acetylation, methylation, phosphorylation, and ubiquitination. Particular combinations of these modifications form “histone codes” that influence the chromatin folding and tissue-specific gene expression.
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The extent of chromatin compaction can be studied by staining chromatin using specific DNA binding dyes. Under the microscope, the dense-compacted regions that take up more dye are called heterochromatin. Heterochromatin is further classified into two forms – constitutive heterochromatin and facultative heterochromatin.
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In 1928, a German botanist Emil Heitz observed the moss nuclei with a DNA binding dye. He observed that while some chromatin regions decondense and spread out in the interphase nucleus, others do not. He termed them euchromatin and heterochromatin, respectively. He proposed that the heterochromatin regions reflect a functionally inactive state of the genome. It was later confirmed that heterochromatin is transcriptionally repressed, and euchromatin is transcriptionally active chromatin.
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Drosophila histone locus body assembly and function involves multiple interactions.

Kaitlin P Koreski1, Leila E Rieder2, Lyndsey M McLain3

  • 1Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599.

Molecular Biology of the Cell
|May 14, 2020
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Summary

The histone locus body (HLB) forms at histone gene clusters. In Drosophila, endogenous genes sequester factors, preventing HLB assembly and expression on modified transgenic arrays.

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Area of Science:

  • Cell Biology
  • Genetics
  • Molecular Biology

Background:

  • The histone locus body (HLB) is crucial for replication-dependent (RD) histone mRNA production.
  • The Drosophila melanogaster genome contains approximately 100 tandem copies of RD histone genes.

Purpose of the Study:

  • To identify sequence elements essential for HLB formation and histone gene expression in Drosophila.
  • To investigate the role of endogenous histone genes in regulating HLB assembly and function.

Main Methods:

  • Utilized transgenic gene arrays with 12 copies of the histone repeat unit.
  • Created a modified array (12xPR) replacing H3-H4 promoters with H2a-H2b promoters.
  • Assessed HLB formation and gene expression with and without endogenous histone genes.

Main Results:

  • The 12xPR array failed to form an HLB or express RD histone mRNA when endogenous genes were present.
  • In the absence of endogenous genes, the 12xPR array successfully formed an HLB and expressed RD histone mRNA, rescuing lethality.
  • The HLB formed on the mutant array contained wild-type factors, including CLAMP.

Conclusions:

  • Multiple protein-protein and protein-DNA interactions are vital for HLB formation.
  • Endogenous histone gene copy number influences the assembly and expression of transgenic histone arrays by sequestering regulatory factors.