P element

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The P element or P factor is a DNA transposon that occurs in the fruit fly Drosophila melanogaster and is widely used in science for mutagenesis . It can change its genomic locus with the help of a transposase .

construction

The P element is a 2907 base pair- long DNA sequence and has at its ends two 31bp-long inverted repeats and two 11bp-long direct repeats , which serve as the recognition sequence for the transposase. It has four exons that code for a transposase, making it an autonomous P element. The approx. 2500 bp long pre-mRNA is spliced to mRNA and then translated . In the germline, all three introns located between the exons are cut out of the pre-mRNA so that a complete translation to the approx. 87  kDa transposase is possible.

An alternative splicing mechanism in somatic cells has the effect that the third intron (between exon 3 and 4) remains in the mRNA, whereby the translation in this intron is interrupted and a non-active (truncated) transposase is produced. This polypeptide is 66 kDa in size and due to its longevity (also) accumulates in the germ cells. Over time it successfully competes there with the transposase and thus functions as its repressor. A stable genotype is created in the gonads. Drosophila strains which have P elements are called "P-strains", the others are called "M-strains".

history

The P element is a very successful transposon. Studies indicate that D. melanogaster did not develop until the 1920s . In 2000, however, there were no more M strains to be found in the wild. The sequence of the P element is identical to elements of the evolutionarily distant Drosophila willistoni group, which indicates the infection via horizontal gene transfer by a species of this group. Thus in 80 years the P-strain has "out-competed" the entire populations of Drosophila melanogaster .

Hybrid dysgenesis

If you cross a male of a P strain with a female of an M strain, the repressor is missing in the newly created female germ line cells and the P element jumps at a very high rate, which is why mutations occur. On the other hand, if you cross an M-male with a P-female, normal larvae develop because the egg cells have the repressor. This effect is known as hybrid dysgenesis .

P element mutagenesis

In genetic engineering , the P element can be used to generate mutations and then to make them molecularly visible.

Two strains are bred for this purpose: strain 1 has a modified P element in which the transposase has been replaced by a marker gene . Strain 2 has a P element with a transposon, but the inverted repeats are defective, so that the element cannot jump, but can develop the transposase. This is then also called a jump starter element (Js).

If the two strains are now crossed, the transposase from strain 2 is expressed in the cells of the daughter generation and this causes the marker P element from strain 1 to jump. The marker gene shows that a transposition has taken place and also the chromosome region where it jumped to. If a gene is hit, genes can be localized using the mutations and the marker gene.

literature

  • Griffiths, AJ (2005). An introduction to genetic analysis. Macmillan.
  • Brennecke, J. et al. (2008). "An epigenetic role for maternally inherited piRNAs in transposon silencing". Science 322: 1387-1392. doi : 10.1126 / science.1165171 .