Stress: transposon turn-on
Transposons are DNA sequences capable of "jumping" from one genomic location to another. One type of transposon uses the enzyme tramsposase to move about the genome, while another known as a ‘retrotransposon’ encodes two enzymes, reverse transcriptase, which transcribes the mRNA of the transposon into DNA, and integrase, which then integrates the transposon into the genome. Ty5 is one such retrotransposon of S. cerevisiae, and for a while it has been observed that Ty5 inserts preferentially into a non-transcribed region of the genome near the telomeres.
This year the mechanism behind this specific integration was discovered: it was found that, under normal conditions, one amino acid located in the ‘targeting domain’ of integrase becomes phosphorylated, and it is this phosphorylation that is required for the transposon to be inserted into the heterochromatin where it will not damage the gene-coding sequences.
But what is really interesting is that under conditions of stress, it was observed that the integrase was not phosphorylated, and consequently, the transposons were not inserted into the telomere sequences but rather, into transcribed DNA, where they caused mutations.
It seems then, that the preference for integration is controlled by the cell and not the transposon. But why would a cell want to mutate gene-coding sequences? The fact that stress caused changes in the specificity of integration of transposons effectively demonstrates the increasingly accepted notion that the induced mutation increases genetic variation upon which selection may operate, thereby increasing the chances of adaptation.
by Alicia Grealy
Student number: 41196504
References
Primary:
Ebina H & Levin HL, 2007, ‘Stress management: how cells take control of their transposons’, Molecular Cell, vol.27, pp.180-181.
