Alu sequence

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Alu sequences are a family of repetitive (repetitive) DNA sequences in genomes from primates . They belong to the short interspersed nucleotide elements (dt. 'Short, distributed nucleotide elements '), abbreviated to SINEs . Alu sequences are each around 300 base pairs (bp) long, but make up over 10% of the human genome (this corresponds to a number of copies of over 1 million). They are particularly common in the above-average rich R bands . Alu sequences are transcribed by RNA polymerase III, but not translated , so RNAs are formed, but these are nottranslatedinto proteins .

The sequence was discovered in humans in 1978 by Catherine M. Houck and her colleagues. It was after the restriction enzyme Alu I (from A rthrobacter lu teus named) because it separates this section into two parts. A 170 bp and a 130 bp element are produced.

Distribution of Alu sequences (green) in the human genome. These sequences are particularly common in gene-rich sections of the chromosomes. DNA is colored red so that gene-poor regions are also visible.

construction

Structure of DNA

Alu sequences are internally duplicated, which means that they have a 5 'part and a 3' part that are related ( homologous ). The 3 'part contains an additional 31 bp long sequence. Each monomer always ends with an on adenine - nucleotides rich sequence. In the 5 'part there are 2 motifs (A box and B box) which represent an RNA polymerase III promoter . The 3 'half is missing the B-box thus has no promoter.

Alu sequences are always surrounded in the genome by two short (7–20 bp) identically structured and aligned sequences (' direct repeats '). This is taken as a sign that they are still transposable , i.e. that they can be replicated by retropositioning them in the genome.

Structure of the Alu RNA

The dimeric structure of the DNA also results in two similar structures in the RNA, which are separated from one another by the adenine-rich sequence of the 5 'part. In each monomer, the RNA single strands form double strands with themselves, which then form hairpin shapes (' hair pin '), but incorrect pairings form loops (' loops '). Each monomer forms a conserved region at the 5 'end in which the bases differ only minimally between different copies and species . The 3 'end of each monomer, however, is variable.

evolution

Alu sequences are dimers, i.e. they consist of two very similarly structured units. The monomers are again homologous to a gene for the 7SL-RNA , but have a 141 bp long deletion in comparison to these.

The 5 'unit at the beginning comes from a monomer family that is referred to as FLAM ( free left alu monomer ), the 3' unit from the FRAM family ( free right alu monomer ). In rodents ( B1 element ) and pointed squirrels (Tu type II SINE), SINEs occur that can be derived from a gene for the 7SL-RNA. However, these are always monomers and can be derived from a common ancestor with the FLAM family. The FRAM family is only found in primates.

Alu -related sequences have also been discovered in the area of ​​viral tyrosine kinase Src mRNA (v-Src mRNA). Therefore, their distribution may not be limited to vertebrates.

function

Recent studies indicate that Alu -RNAs bind to the mRNA-producing RNA polymerase II and to promoter regions for protein-coding genes under the influence of heat shock and thus inhibit transcription.

Individual evidence

  1. ↑ a b J. O. Kriegs, G. Churakov, J. Jurka, J. Brosius & J. Schmitz. Evolutionary history of 7SL RNA-derived SINEs in Supraprimates In: Trends in Genetics 23 (4) / 2007, pp. 158–161 doi : 10.1016 / j.tig.2007.02.002
  2. ^ MA Batzer and PL Deininger. Alu Repeats and Human Genomic Diversity. Nature Reviews Genetics 3: 370-9 (May 2002)
  3. ↑ a b c P. D. Mariner, RD Walters, Ce. A. Espinoza, LF Drullinger, SD Wagner, JF Kugel & JA Goodrich: Human Alu RNA is a modular transacting repressor of mRNA transcription during heat shock In: Molecular Cell 29/29. February 2008, pp. 499-509
  4. ↑ CM Houck, FP Rinehart, CW Schmid. A ubiquitous family of repeated DNA sequences in the human genome in: J. Mol. Biol. 132: 289-306 (1979), doi : 10.1016 / 0022-2836 (79) 90261-4
  5. ↑ CW Schmid & WR Jelinek: The Alu family of dispersed repetitive sequences In: Science 216/4. June 1982, pp. 1065-1070
  6. ↑ Y. Quentin: Fusion of a free left Alu monomer and a free right Alu monomer at the origin of the Alu family in the primate genomes . In: Nucleic Acids Research 20 (3) / 1992, pp. 487-493
  7. ^ Y. Quentin: Origin of the Alu family: a family of Alu-like monomers gave birth to the left and the right arms of the Alu elements . In: Nucleic Acids Research 20 (13) / 1992, pp. 3397-3401
  8. ↑ Czernilofsky, AP. et al . (1980): Nucleotide sequence of an avian sarcoma virus oncogene (src) and proposed amino acid sequence for gene product . In: Nature 287 (5779); 198-203; PMID 6253794 ; doi : 10.1038 / 287198a0
  9. ^ CW Schmid, WR Jelinek (1982): The Alu family of dispersed repetitive sequences . In: Science 216 (4550); 1065-1070; PMID 6281889 ; doi : 10.1126 / science.6281889

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