The human genome was shaped by an evolutionary tug-of-war, as elements of the primate genetic sequences battled with a type of protein over the blueprint of life.
Retrotransposons are DNA sequences that are able to move from one section of a genome to another, earning the structures their nickname of "jumping genes." They are controlled by genes that play an important role in the struggle. Biologists believe these structures were originally independent viruses long ago and became entwined in primate bodies, long before the evolution of the earliest human beings.
Repressor proteins that "shut down" jumping genes were first identified by researchers from the University of California Santa Cruz. Investigators found these evolved quickly in primates, as they fought to repress the expression of retrotransposons. Transposable elements, like retrotransposons, make up more than half of the human genome. Each time repressor proteins found a new way to suppress the actions of the genes, the structures would evolve a new method of working around the proteins.
"We have basically the same 20,000 protein-coding genes as a frog, yet our genome is much more complicated, with more layers of gene regulation. This study helps explain how that came about," said lead researcher Sofie Salama of the UC Santa Cruz Genomics Institute.
Jumping genes are only able to replicate within the genome of a living being. As reproduction takes place, healthy genes can be displaced by the retrotransposon. Most of the time, these changes make no difference to the overall health of the individual host. Once in a great while, these adaptations prove favorable, enhancing the expression of beneficial sequences. Often, however, these modifications create unhealthy changes to the genome. Therefore, evolution would have favored the development of repressor proteins, in order to keep this danger under control.
Investigators discovered KRAB zinc finger proteins, which repress the expression of specific genes. Over 400 genes in the human genetic code battle with these newly discovered structures, 170 of which developed after the evolution of the first primates. Many of the older designs of repressor genes are now utilized by primates for other purposes within their genomes.
"The way this type of repressor works, part of it binds to a specific DNA sequence and part of it binds other proteins to recruit a whole complex of proteins that creates a repressive landscape in the genome. This affects other nearby genes, so now you have a potential new layer of regulation available for further evolution," Salama told the press.
Mice were used in the study, since they lack the zinc finger proteins that are found in primates, including humans.
Study of the ancient struggle within primate genomes was detailed in the journal Nature.
