Alu element

Alu sequences are a family of repetitive ( repetitive ) DNA sequences in the genomes of primates. They belong to the short interspersed nucleotide elements ( dt, short, distributed Nukleotidelemente '), SINEs abbreviated. Alu sequences are each about 300 base pairs ( bp) long, make it about 10 % of the human genome from (this corresponds to a copy number of about 1 million). They are especially common to find in the above-average gene-rich R- bands. Alu sequences are transcribed by RNA polymerase III, but not translated, so it RNAs are formed, but does not translated into proteins.

The sequence was discovered in 1978 by Catherine M. Houck and their counterparts in humans. It was named after the restriction enzyme AluI ( from Arthrobacter luteus ) because it separates this section into two parts. This produces a 170 bp and a 130 - bp element.

Construction

Structure of DNA

Alu sequences are duplicated internally, which means they have a 5 'portion and a 3' portion that are related to each other ( homologous ). Here, the 3 'portion includes an additional 31 -bp sequence. Each monomer always ends with a rich adenine nucleotides sequence. In the 5'- part 2 motifs to find (A- box and B box ), which represent an RNA polymerase III promoter. The 3 'half lacks the B box thus has no promoter.

Alu sequences in the genome are surrounded always by two short ( 7-20 bp) identical construction and rectified sequences (, direct repeats '). This is seen as a sign that they are still transposabel, that can be duplicated by retroposition in the genome.

Structure of Alu RNA

By the dimeric structure of the DNA also arise when two similarly constructed RNA structures which are separated from one another by the adenine-rich sequence of the 5'- portion. In each monomer, the RNA single strands form duplexes with themselves, which then form hairpin forms (, hair pin ' ), however, not suitable pairings form loops (, loops ') made. Each monomer forms at the 5 ' end of a conserved region, the bases differ only minimally in between different copies and also types. The 3 'end of each monomer, however, is variable.

Evolution

Alu sequences are dimers, thus consist of two very similarly constructed units. The monomers are again homologous to a gene for the 7SL RNA, but have a 141 bp deletion when compared to these.

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

Alu - related sequences were also discovered in the field 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 under heat shock influence on the mRNA -producing RNA polymerase II and promoter regions of protein coding genes and thus inhibit transcription.