LTR element
LTR elements are DNA sequences that are surrounded by so-called long terminal repeats, which are necessary for integration into the genome . These include endogenous retroviruses and LTR retrotransposons .
Importance in evolution
Due to the repetitive sequences, LTR elements can easily be cut out of the genome, multiplied and reintegrated at any point in the genome. As a result, the insertion of an LTR element can increase or decrease the activity of the genes in the environment, change the tissue specificity or create new gene products through alternative splicing . They therefore make a significant contribution to genetic variability and thus to the evolution of organisms. In 2005, 20 human genes were known to be controlled by viral LTRs. A total of at least 600,000 LTR elements were found in the human genome.
For example, there are five amylase genes in the human genome . Two of them are active in the pancreas and three in the salivary glands , which open into the oral cavity. Many other mammals do not have amylase activity in saliva . They also have the genes for amylase in the cells of the salivary glands, but these are tissue-specifically inactivated. By integrating an LTR sequence from a retrovirus, the amylase genes in humans could also be activated in the salivary glands of the mouth, making starchy food easier to digest.
Another example is the control of the level of fat in tissue by the hormone leptin . The genes for the hormone and its receptors are likely controlled by HERV-LTR retrotransposon sequences, which in turn are activated by steroid hormones .
Individual evidence
- ↑ H. Donner, RR Tönjes, RE Bontrop, R. Kurth, KH Usadel, K. Badenhoop: Intronic sequence motifs of HLA-DQB1 are shared between humans, apes and Old World monkeys, but a retroviral LTR element (DQLTR3) is human specific. In: Tissue antigens. Volume 53, Number 6, June 1999, pp. 551-558, ISSN 0001-2815 . PMID 10395105 .